wn has released QuickBoot Test 8... Still in the testing phrase, QuickBoot lets you conveniently boot from one game to another, whether that be an ISO, POPS, UMD, or homebrew, without returning to the XMB. QuickBoot installs like any other plugin; however, you'll also require cmlibMenu, which is included.
Copy quick_boot.prx to /seplugins, copy cmlibMenu to /seplugins/lib, and enable the prx in both game.txt and pops.txt.
The plugin is configurable, letting you define QuickBoot menu colours and what button combo brings up the menu. All that in /seplugins/quick_boot.ini.