Okay, I found a way around it. The mighty Google led me to this:
http://www.mediafire.com/file/iz71k0ius
verter.exe
Would appear that someone already had this problem and wrote a converter.
What's not quite so clear is how to use it. You can drag and drop your .sav or .srm file onto the executable in the File Explorer. The file will be converted and will output in the same directory as the source file.
So I did that with my .sav file from RIN (after unzipping it with 7-Zip) and it output as an .srm file. Put it in the relevant directory in Retroarch on my PSP and it all works fine.
Will compare audio soon, although I must say that playing a track in LSDJ on RIN sounded pretty much the same as the .wav file the artist had recorded from original hardware (I got the sng files when I bought their music on bandcamp).
What I *will* say however, is that stereo panning on RIN is completely wrong. Audio designated as left comes out the right channel and vice-versa. I presume this isn't the case with Gambatte.
Anyway, hope the link and info above saves someone some time in the future because, like breakphase, I also use my PSP for convenience (commuting on several trains means the sleep functionality of the PSP is a godsend!).