How about a BTC address.NMC address as user id. If a NMC address is not provided then more for you or split between those who do care for them.
Many miners can't send usernames that long. If you export your Bitcoin key and import it to Namecoin, that works reasonably.
Can it be expected that every single user will do this? If no will coins go to waste using this method? How about a NMC Address in the password field?