The variable "keystore" is the keys(pubkey pubkey_hash privatekey etc.) of your wallet. That means if you submit a block, you should sign it up with the pubkeys from your wallet,  but  username-as-address mode just only sends p2pool your pubkey_hash.  you can easily get a pubkey_hash with an address, but never get pubkey, just like you want to get the text password from md5.

I've already found a tricky way to fix this problem, just wait for the next version