Are those keycaps backlight friendly?
How do they feel with o-rings?
No, and the same as other keycaps.
As with many things, "backlight frendliness" is a matter of subjectivity, and -judging my pointyfox's response here- much debate. I like imsto's thick PBT over backlighting, and think it looks superb. Sometimes I'll leave the green backlighting on and enjoy the effect.
o-ring compatability is mostly an issue with keycaps that have short stems (cherry F and J keys) and some SP keycaps. I
think it's fine with imsto's PBT, but cannot confirm as I've yet to actually install my newly-rediscovered o-ring sampler pack.
You should buy a Poker II as a mod board and start tearing it apart. Build or buy some ergo clears or something and solder those on, solder on some led lights. The stock Poker II caps are really nice so you wont have to buy new caps for it right away.
I would suggest a keyboard without a plate to be a better mod board, as you can change switches, stems and springs more easily that. Try to get a non backlit pure and add your own LED's and "tune" your switches to your preference. The poker II keycaps don't let much light through, so your mileage may vary.