This sounds like the kind of feature which is best implemented by a patch to the Rockbox playback/playlist code, rather than a plugin.
If I understand correctly, you want Rockbox to play a track at random, then pause for a random amount of time, and then play another track at random, and so on ?
So assuming you're happy with Rockbox's existing shuffle and playlists features to pick your songs at random, all that you would appear to need to do is to add the random-length pause between tracks.
The place to look to do this is apps/playback.c Sorry, I can't be any more specific - I don't know that code very well.
Regarding documentation - there is a fair amount of developer-related documentation in the wiki (see the For Developers section in the index), but there is never enough. The fact of life is that most Rockbox contributors prefer to code instead of writing about coding.
The best place for answers to specific questions about how Rockbox works is the IRC channel.
I'm curious to know why you want such a feature though.