TBH, I was hoping that my version bytes would become the standard. Hypothetically, if my implementation were actually used by alt-coiners then when drafting the spec it'd be more of an issue that the spec has to deal with (that people have already adopted a standard to use). Unfortunately, there's just no BIP-style development for any of the other coins, so I was forced to improvise if I wanted to support altcoins.
I was indeed going with XXpv/XXub for the public/private keypair prefixes. For testnet, I was a little less concerned about consistency but it seems you have gotten the general gist of things.
The code isn't set in stone. If you guys want the version bytes changed, let me know:)
I like the mainnet convention you're using. It's probably the most human-readable to be done in 4 characters. Would you consider changing the testnet prefix to XXtv, XXtb for private and public extended keys, respectively?