there are racers using NIMH combinations in there stuff because they are worried about lipo fire issues and see a benefit to the new LiFePO4 technology. I don't agree with them as far as there concerns with regular lipo as I run both softcased and hardcased packs in 1/8 electric down to 1/16 electric, but it doesn't mean they are not out there.
As for the various classes being ran, at least to me I think you have to start somewhere as other techologies have (I think brushless started first in mod correct?).
Initially this could be done in that manner or something similar (keep max voltage limits the same), and then let the racers decide with there wallets if its viable tech. Others probably have better ideas on how to do this.
And in my case, at least from what reports I have gotten from those running the packs in classes like 1/8 electric and 1/10 offroad electric mod, they do work well when matched with the appropriate motor and gearing. It could possibly apply to other classes, or it might not. But, its an option I wouldn't mind seeing them allowed within the current framework of rules, in particular the maximum voltage limits for the various classes.