I don't know if this has already been asked, but I wonder if there's ever a way to make BW playable in Single Player (not Instant Action) mode. It will be a lot more fun than just in Instant Action mode.

I think it's not possible because like the first UT all weapon customization mods are only in multiplayer modes like the instant action and the multiplayer
Gunreal have it's own sigle player mode but Gunreal have a separate system.
I'm not certain but by using the cheat code we can summon the ballistic weapons but it's a cheat, no ?

There should be some kind of configuration for UT Single Player mode, as whatever the single player game it is, the in-game interface should still act as a local game server... (similar to CS Condition Zero's mission mode. Addons like AMXModX can work in it)

EDIT: I wonder what does the ServerPackages in UT2004.ini mean. It looks like BW is separated from original UT, thus BW is only enabled in your custom modes by default, and can only be activated by mutators to make it work for default modes, including Assault. I wonder if BW is also made of packages. Are packages in the format of ".u" or something in System folder?

If it was possible to make UT2004 engine to load the BW files by default, and apply all configurations the user has made, the BW should be automatically loaded when playing Single Player and other modes. But doing this could probably make it impossible to unload, if someone want to play original UT2004 modes and weapons.