So I've gotten a few PM's from people asking how to do it so though I'd tell you all(that is if you did not know).First what you need is the opening_plugin.prx from your respective firmware(I will provide the modded PRX's also) and open it in a HEX editor.You will be looking for the offset 666C..or for the text string flash0:/vsh/resource/gameboot.pmf.Place your cursor at the beginning of the F in flash 0 and type the following text string:ms0:/PSP/SYSTEM/BOOT/.If done right the T in BOOT should finish before the backslash right before gameboot.pmf.Then just flash the new opening_plugin.prx to flash0:vsh/module(module folder may be hidden..look in folder options)Now place the gameboot you would like to use in a folder name BOOT and place the BOOT folder in ms0:/PSP/SYSTEM.Now you will be running a gameboot from your MS.
***NOTE***
The Longer Gameboot RCO's that I made a thread about will still work..just put opening_plugin.rco in the resource folder in flash0 and put the gameboot in the BOOT folder.Enjoy.
PRX Package