After alot of trial and error, I have gotten SOCOM to work on the 1.5 Firmware... The only current limitation is multiplayer won't work (ok, a big limitation) but you can still have fun playing all the single-player campagns without having to upgrade your PSP....
First of all, I leveraged alot of info from the PSPinfo forums to get me started to kudos to them... Here is the process I followed (the first few steps are identical to getting any normal (non socom, non GTA) 2.X game working on 1.5 with some changes that I found:
Extract the SOCOMiSO image with WinRAR to a directory on your PC
* Go to this directory, and remove the SYSDIR/UPDATE folder
* Go to USRDIR and remove all the languages you dont want (keep e_text.afs for English, deleted the others)
* Copy the SYSDIR/EBOOT.BIN to your PSP, in a folder called "Enc" at the Root
* Copy the 2 folders decrypt and decrypt% (from HERE) into your PSP/GAME folder
* Run the Decryptor v0.1 from the PSP (obviously, you have to be in 1.5 for that)
* This produces an EBOOT.BIN file in a new folder called "Dec"
* Rename this file into BOOT.BIN, and overwrite the original BOOT.BIN in SYSDIR with it
***** HERE is where we do things slightly differently***** Normally we'd replace the SOCOM PRX files with PRX files from a 1.5 game - unfortunately no other 1.5 game uses half the libraries that SOCOM uses so we'd normally be up **** creek without a paddle.... however:
The SOCOM BETA that was released a while ago didn't have a 2.X and above restriction (I.e. it could be played on 1.5) leading me to guess that I could substitute the SOCOM PRX files with the SOCOM beta PRX files....
Acquire an ISO of the SOCOM beta (go on, you know where to get it) and replace the SOCOM USRDIR\RUN\PRX dir with the same dir from the BETA.
Finally, generate a new ISO file from your modified SOCOM directory structure using UMDGen or VCDROMx or your ISO generator of choice and Viola! the SOCOM game should load using UMD Emulator 0.8C with Copy Load mode (with 'use BOOT.BIN' turned ON).
Once again, the WIFI doesn't work for some reason (I have a few more things left to try) and another small annoyance is you can't save your profile to memory stick - but this should at least allow folks who own SOCOM but have shyed away from upgrading to give it a whirl!