It's proven that booting from MS is possible. However, I guess the bootable file in MS must be encrypted. (AES?)To use Game Sharing in Namco Museum, the player with the game selects the transmit function from the game's menu, while the other player selects Game Sharing on the main menu of the PSP (it's the icon with two PSPs and arrows between them indicating sharing.) Once the systems recognize each other, a short download starts where the game-less player is sent the playable selection -- after that is complete, the gamer can go ahead and play without even having a UMD game in the system.
For the Namco Museum feature, all seven classic arcade games are uploaded in one big package -- Pac-Man, Ms. Pac-Man, Galaga, Galaxian, Rally-X, New Rally-X, and Dig Dug -- and players can choose and play any one of them from a simple menu without having to download anything more. The cool new Remix modes are unfortunately not available for Game Sharing, there is curiously no use of the Game Sharing for multiplayer gaming, and the seven arcade classics are stored to RAM instead of to Memory Stick -- once the PSP is shut down or sent to the Home menu or another game is played, the sampler download is erased.
Now, it seems that PSP can download image to RAM directly and, run it.
Possible cases :
1) the image is not encrypted.
2) the image is encrypted, and client PSP decodes the stream before storing into RAM.
3) the image is encrypted, and client PSP decodes it run-time.
I think 3) is not likely because, the case leads several issues hard to handle. (ex : every memcpy operation needs decryption. OS must keep encrypted memory blocks isolated from decrypted. blah blah ...)
Though I think 2) may be the case, I guess 1) still have high possibility because, I'd heard that SCE library does not provide AES related operations to developers.
So, if we could intercept and find out what game sharing packets contain, we may find the way to upload custom program into PSP.
PS. Some unofficial comments from official PSP developers : SCE has plan to include web browser to OS but, it may be able to access only official sites. ( mp3 downloading? ) SCE seems that they don't want to allow high-quallity movies on MS. (resolution restriction of MP4 is intentional) SCE never allows 3rd party developers to store any executables into MS.
