You need a custom PSP firmware that'll play unsigned EBOOT's. I have no clue how that works on the PS3 emulating a PSP, but maybe this explanation will help you find a way? (If you do, then please report back for others to know.)Following your comment, I tried rebundling the ISO with a signed EBOOT.BIN to no success, but among the things I've tried:
1) Fixing the LBA table (somehow something was wrong with EBOOT.BIN's LBA position?).
2) Replacing the patched EBOOT.BIN with the original EBOOT.BIN.
3) Sign EBOOT.BIN with PrxDecryptor.exe in PSP2PS3.
4) Sign EBOOT.BIN with seboot.exe in PSP2PS3.
5) Sign EBOOT.BIN with sign_np.exe.
Solutions 1, 3, 4 and 5 will boot all the same, but won't save.
Solution 2 won't boot at all.
For those interested, this is on a Rebug 2.82 CFW PS3.
I tried all of these with the "PSP Remasters Launcher" and I'll try again with the other available Launcher since some subtle differences exist between the two.
By the way, a subject came up while researching the issue: it seems that some ASM hacks won't work on the PS3. So just in case, I'm asking: are there any chances that some changes were applied around the syscall initiating the saves?
Thanks for the help