OK, I'm going to go ahead and answer part of my own question from a few days ago.
I got Stand O'Food the other day. It is a very good conversion of the PC game on the PSP...and it is playable on PS3 as well!
When I first got it, I played on PSP and PS3 separately, with a save file for each one. I decided to go with my PSP save as my "main" save. Earlier tonight, I backed up the PS3 save file onto a USB flash drive (very interesting how it saves it there...more on this shortly), and subsequently erased it in preparation for transferring the PSP save file.
I connected the PSP to my PS3 via USB, and went to the Minis save data folder on PS3. It recognized the connected PSP, and all 5 of my game saves from the Minis I have on PSP (Stand O'Food, along with my other 4 Minis which still aren't playable on PS3
yet). Copying the data was very simple, just like copying most ordinary PS3 save files from a USB flash drive back to the hard drive (if you've ever done that).
I then launched Stand O'Food on PS3, and sure enough, the data was recognized, and I successfully resumed right where I left off on PSP earlier. So, I can confirm that it is possible to migrate Minis save data from PSP to PS3!
I have not tried transferring from PS3 to PSP yet.
There are a couple of interesting observations I've made:
1. The original save file was 160 KB on PSP, but the one generated by PS3 showed up as only 60 KB. After transferring my PSP save over to PS3, it appeared as 61 KB on PS3. After playing with that data and then saving the game, the file is now 62 KB. Meanwhile, the file on PSP has been 160 KB every time I checked. I assume that the PS3 is compressing the save data file, which might account for large difference when compared to PSP, as well as the slight fluctuation in size between different play sessions.
2. When transferring my original PS3 save file to USB flash drive (the one that was generated by the PS3), the system did not put it within the \PS3\SAVEDATA folder like most PS3 save files. Instead, it created the folder \PSP\SAVEDATA and placed it in there.