I know that this thread is old but since I bought the game recently and stumbled over the same problem I'd like to post one possible solution for it (since the game makes much more fun with mousewheel).
1) Download the free program GlovePIE.
2) Extract it anywhere you like and start the executable (no installation necessary).
3) Just type those two lines in the window
4) Klick Run to run the script.
5) Start Blood 2
6) Assign the key "r" to "Next Weapon"
7) Assign the key "t" to "Previous Weapon"
8) Play the game
After quitting the game stop the GlovePIE script.
Instead of "r" and "t" you can use any keys you want to use for rebinding to the weapons/mousewheel.
This worked for me.