Possibilities I see:
- Send to the Bitcoin key on the Namecoin network (ie, key export/import) on demand
- Require miner that can send a long username
- Provide a web interface for account options (including Namecoin rewards and changing minimum payout)
I like the lattemost the best

Option 2 would keep things simple and registration-free, but if that doesn't work, Option 3 (think that's what Slush went with) is next best
