ffgl itself cannot access any controls internal to the host, ffgl doesn't even know what 'resolume' is. As far as ffgl is concerned it's just a self sufficient plugin that gets hosted by some software which in our case is resolume, but there may be other hosts.
You could however implement your ffgl however you see fit. So if you've got a way to get the information you need over osc you could build this into your ffgl plugin. ffgl 2.0 does support dropdowns and faders as well, you'll just need a way to populate those yourself.
One thing you'll notice if you choose this direction is that ffgl parameters are static and cannot be changed once they're created, so you cannot dynamically populate the dropdown depending on what layers are present in the current composition, sorry