If indeed it is just one eboot then you are going to have to go into more traditional fields of ROM hacking and it will involve going into assembly hacking (generally regarded as the hardest part of hacking), though the simpler side of that. We normally see such things when people want to have game collections like the 2 in 1 ones on the GBA, or other compilations or minigames but the idea is the same -- at some point after boot there will be a branch to the game code, however you normally can't jump straight there as there will have to be some setup done first. You then either find a point after the setup has happened (most times it is fairly basic so do it after a second logo or just before the title screen and that is probably more than enough) to insert your jump or you replicate the setup. Not all games will, some games then containing multiple executables and a frontend to select them, but as there is only one eboot and given the way the source engine works it is not likely to be one of those exceptions, and even if it was that then you could still edit the selection frontend to auto jump to that.
Bonus for the finding the branch code is it can be forced to happen very easily, tends to happen soon after boot and probably won't have much else going on at the same time, though I mainly do it on systems like the GBA and DS which have extensive emulation and debug options where the PS3 is a bit more limited if you need to step outside basic memory bothering and static decompilation.
As far as ripping assets (using the more classical definition of chopping out parts of a game to reduce size rather than the more modern usage where you take the textures/pictures/text/audio to play with somewhere else) then you can do it, however going by what happened on other systems (see all the complaints with pirated versions crashing, mods not working with pirated versions and working around it by not looking at something which was a missing (ripped) texture/model) it might be easier said than done. On the other hand you could probably look at what was done to make a working version of the pirated games (there are various standalone ones) and see if there are any correlations between files in the PS3 version and the PC ones. I don't know what sort of space it will gain but if you go on your chosen pirate site and look up the sizes for complete orange box and standalone portal you might get an idea.
