Random thought : if I replace the LED with a small cheap laser, I could get a higher light density (hence rising le the tension higher). On the other hand, that would go against the original idea to make an MCU programmer that you could built with part you may already have or could buy for cheap at local store.

Anyway, I ordered 10 laser pointer (for $0.16 each) on ebay. I'll try using them if I can't get the programmer to work with LEDs.