For those that want it, here is a version of Painkiller.exe which supports listing SP maps for MP loading!
This is not essential for PKBeta testing, but useful if you like doing everything through the dedicated server interface / listen server menu ui. It goes without saying that this also does not require the game cd, and is fully compatible with Vanilla Painkiller.
If you are feeling brave you can replace your existing Painkiller.exe by unzipping renaming this file. Remember to backup. This will not be overwritten by Steamfish.