I may end up with a currently non-functioning LOA undercabinet fixture. It takes F15T8 lamps. I could probably put an F15T12 in there as well. My question is has anyone converted one of these to preheat when the cheap circuit board "ballast" craps out? It looks like a preheat choke would fit inside. I could drill a hole for a starter socket and use one of those clip-in style starter sockets.

I've done it with Progress Lighting under cabinet fixtures designed to take one F20T12 lamp. They came with Keystone trigger start magnetic ballasts that never made it under the cabinets. I have converted them all with preheat chokes and clip in starter sockets. I had to drill and file holes for them, though. They work quite well although I have to change one ballast because of noise. Oh, and I need a new lens for one that was broken in shipment and I noticed too late to lay a claim. I'm sure you will succeed.

The plastic GE branded F15T8 ones with a electronic circuit board ballast were originally offered in preheat. They had a molded mount where a stripped down choke core would fit into and I guess it used a hardwired glowbottle starter like some of the newer preheat undercabinet fixtures.

Yours might have been originally designed for a preheat ballast too. Take it apart and see it you see mounting points for a choke ballast. I personally don't like hardwired glowbottles but if you have a 7/8" drill bit you can drill a hole in the casing for a starter socket.

Oh yes, I assumed you have a metal one. It it's plastic, it would still be possible however it might be glued/welded shut so you have the problem of opening it. Mine are all metal.

Won't an ordinary choke heat too much for a tight Plastic case ? Good Plastic FL luminaires often use lowloss (longer size) chokes

Here is the actual fixture. I put a lamp in it, powered it up and...nothing. Dead as a door nail. The cover actually comes off very easily. I didn’t see any obvious failures on the circuit board. The board has a total of 6 wires, hot and neutral from the mains (120v), and a pair of red wires running to each lampholder. Looks like some form of rapid start in the way it’s wired. I remember seeing fixtures of this basic design in preheat with a hard-wired glowtube inside the fixture. I can look at the ballast in my 2 foot Metalux preheater and eyeball whether one will fit in there.

See that pair of clips next to the electronic ballast? That's where the preheat ballast was in the original version of this.
I'm not surprised they didn't change anything with the design of these .lol.