Er, since I've gotten no feedback at all, I wrote up some rules regarding starting with higher-level characters or level-adjusted races. I don't know if this is even something you want to allow, but I made a proposal anyway. Let me know if you want to throw it away or modify it.
....which just crashed and got lost. Basically, there are two options: retire an existing character to recreate another at 2/3 the XP, or transfer XP on a 1:1 basis from an existing character without losing the character (like creating a magic item), though only down to the minimum necessary for the existing character to maintain his current level. Level-adjusted races need a judge's approval and are created in the same manner, but - possibly, depending on what the judges think - retiring an existing character may let you swap XP at 1:1 (for total ECL; 1000 XP gets you a 0 XP aasimar or a 2nd level human) for level-adjusted races, because level-adjusted characters are still slightly less powerful than normal races of the same ECL, due to lost level-based benefits, like hit points or feats.