There's no way to auto-run an homebrew at PS3 launch.
And, on the other way, to run game directly from xMMb:
There's very few games working in "auto-run" method from multiMAN,so you need to exit to XMB for most of them.
There is (or will be) a way to select one of the 3 auto-run method.
Currently, the game's boot method is asked when you select the eboot.bin of a game in the file manager mode.
Maybe there will be a proposer "select which method to use" menu in the game's sub-option page.