They appear to be... Archives? Opening "rls_xpk_game.xpk" in Notepad++ shows some recognizable things amongst the 'NUL' boxes. ie, bluecar.tga, samplecube.bcmdl, usershader.bcsdr Some recognizable formats, along with some more mentions of XPK files. I know like, nothing about software and hex, so I'm clueless here.
Unless I'm completely missing something, or missed something obvious and am going to feel stupid afterwards. But, anyone know?