This time i'm asking before trying 

So i got that semi-pearl, semi-reflector mercury vapor lamp which is absolutely a beauty to display somewhere in my living room. However, at full blast it'll draw over 400w total, as well as produce enough light to light a whole section of my neighborhood.

Are there ways to run it on high voltage, low current, like a cold cathode tube - or do these ones also rely on thermionic emission and therefore require the full power and only the full power, to avoid sputtering and cathode coating breakdown?

My goal is just to have it light up a bit. I will be getting the original ballast, so i can always run it full power, but that's a bit much in the living room.  The alternative is shining a few 365nm LEDs at it - that will activate the phosphor in the reflector part, creating a truly beautiful red glow (see picture). However, it goes without saying i prefer using the lamp itself and only running it at full blast when i'm hosting a block party or something late at night.

EDIT: i can actually make it glow no contact on my little tesla coil. The electrodes are not carrying any current, the gas ionises on RF fields alone. Seems like a good way? Unless there's some physics going on in the arc tube with the metals present that i have no idea of?

Running a HPM lamp with a glow discharge will inevitably kill the electrodes rather quickly, just because of sputtering.

Some other choices to have that pretty pink glow is to get a red CFL or CCFL tube, pink sort of T8 fluorescent (these are made fluorogermanate, more dirty reddish color, or red europium phosphor - you want that one) or have a neon maker made you a tube with a 'coral pink' phosphor.

A fun idea may be to stuck some small germicidal lamp into a HPM envelope

 

Actually, the lamp you have here is probably fluorogermanate coated as it reacts strongly to UVA. You can mount a strong small UVA source (a small blacklight tube or high power UVA LED module) just behind it to make this pretty deep red ruby glow permanent, without any wearing of the collectible lamp.

A fun idea may be to stuck some small germicidal lamp into a HPM envelope

I could do that with one of the completely worn out HPL-N 80w's i'm getting sooner or later, but not with this huge thing. There's a decent risk of the outer bulb shattering, and i wanna keep it in working order.

I've chucked a bunch of 365nm LEDs in my Aliexpress basket. I'll see how well i can make that work.

Actually, the lamp you have here is probably fluorogermanate coated as it reacts strongly to UVA. You can mount a strong small UVA source (a small blacklight tube or high power UVA LED module) just behind it to make this pretty deep red ruby glow permanent, without any wearing of the collectible lamp.

Which phosphors respond to which wavelenghts? I've noticed my modern HPL-Ns barely (if at all) respond to 365nm. That said, it may very well be that the phosphors of those lamps have gotten very worn out by now.

Cutting the lamp neck evenly is a bit tricky. Classic glassblowers tricks are to do a deep circular scratch in the loop across the neck with a diamond or carbide wheel cutter, wet it with saliva or just water and then to apply a local thermal stress - touch with a melted tip of a thin glass stick or tube. Propagate the crack with repeated touches. Or use a loop of nichrome wire of about 0.5mm diameter. Wrap the wire across the glass and heat to dull red for about 10 seconds, then remove and briefly touch the glass with a wet finger. It should crack cleanly because of thermal stress. Or use both circular scratch and nichrome at the same time. The success depends on the type of the glass used. Soft glass with high coefficient of expansion is the easier to cut that way. Hard glasses like boro or alumosilicate may be reluctant to thermal shock because of inherently low COE and may be easier just to carefully cut with a fast rotating thin jeweler's diamond disk with a bit of water to cool.

It may be a good idea to relieve the vacuum inside the lamp, because a fast inrush of air as you cut it may blow off some phosphor. Some ideas may be to start the partial crack but then let the lamp alone for about a hour to leak, and then finish, or get access to the pumping tubulation inside the base/stem, for example with gentle cracking the black vitrite insulator. Melted tubulation tip usually holds a high residual stress, and if heated quickly with a sharp butane torch, will crack and slowly let the air in. Or you may try just to break the tubulation, though there is again some risk of blowing off the phosphor. 

Wrap the lamp body with a thick towel to avoid cuts on mishap. Note that if the area to cut is poorly annealed and holds some stress, it will never cut cleanly whatever care is used. May be only if the whole lamp is re-annealed in the oven. So do not blame yourself if the cut will not work out. Practice on some junk lamps.

Regarding phosphors. First, in a HPM lamp mercury and the discharge do not contact the phosphor until something very bad happens, so it ages to much less extent then in fluorescent lamps for comparison. So it should not be completely dead even in well aged lamps. There are many types of phosphors in HPM lamps, some are reactive to UVA and some not. Modern DX europium-activated vanadate almost does not react. Older Mn activated fluorogermanate strongly light-ups with that pretty ruby color at about 650nm. Not sure about other types, cheap orange silicate will be probably dead in UVA too. Interestingly, as your lamp is the old Philips, it may use a rare red Mn activated arsenate phosphor instead of fluorogermanate in fact!

Cool lamp!