Talking about boot up time savings, it's not specifically directed to this question but, why doesn't Rockbox check for the files in background, leaving users in the main menu and fully operational.
The menu would show up for a second or so, and then if the background code verified the existence of a plugin to be autorun, the plugin would be launched.
What I'm saying is kind of a concept, it doesn't have much technical basement behind it, but anyways I leave here the idea (as this is on the Feature Ideas section... it's on the right place).