On the simulator, .rock are just a copy of the .elf file
To debug a plugin you can do:
% gdb ./rockboxui
(gdb) b plugin_start
Function "plugin_start" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (plugin_start) pending.
(gdb) run
Navigate in the sim, run any plugin, gdb interrupts the simulator and give you the prompt.