As far as I know, every time a new map is made all the bitmaps, sounds, etc... must be compiled into that same cache file. I mean, if you can find where/if bitmaps.map, etc... is being referenced in the new cache file, you could try injecting your new bitmaps into the .map with tools like Eschaton, or if you can get it to work, Swamp Fox's HXE. I've never really looked into it though, so no guarantees.
OpenSauce does offer 'mod sets', if you don't mind .yelo maps. There doesn't seem to be much documentation on utilizing it, but I'm sure it can be worked out with experimentation: https://code.google.com/p/open-sauce/wiki/Doc_Halo1_GettingStarted#OpenSauce_for_the_Game https://code.google.com/p/open-sauce/wiki/Doc_Halo1_ToolCommands#build-cache-file-ex