yes/no, when the xbox red rings, it shuts the system components down for protection. the 12v source should still be active, but I doubt the dvd power is active. so its like 50/50 but you can use it for practice. I would get a rrod console and xclamp it then do your led mods, once all is good and working transplant the led components to the good xbox for long time use....
remeber boys and girls, the number one thing you can do to prolong the life of your led mods is to use the proper resistors. it would be a shame to do all that work and have it burn out in a month or two.