Background: In the audio processing world, most programs to compose and mix music in (collectively called DAW's from now on) are commercial and closed source. These programs can extend their functionality using some common plug-in specifications.
There is one common 'free' and cross-platform specification that is widely supported (Steinberg's VST specification). There are several others, most commonly Apple's AudioUnit and Avid's RTAS- and AAX specifications. AudioUnit, RTAS and AAX are locked in to the companies' proprietary DAW platforms (Logic and Pro Tools).
For whatever reason these specifications were developed, they are nearly completely identical. However, Pro Tools and Logic refuse/don't support loading plugins from any open specifications except their own.
These specifications are so identical that creating a wrapper around all of these specifications is trivial and possible. This, then, has created a mess in which developers of audio plug-ins must separately distribute a lot of permutations of their plugins, but it has become common practice.
Situation: I created a meta-plugin/wrapper, that allows to identify itself as any format (specification) and load any format (such that you can load VSTs in Logic, for instance). The plugin is free and licensed under the GPL.
It works completely on its own. But it can, at run-time optionally load any end-user provided plug-in, or, optionally, save an emulated 'copy' of the loaded plug-in disguised as any other format. This last feature allows the unsupported format to be loaded seamlessly in any other locked-in proprietary host (but under the hood, it is still wrapped through my plug-in, just statically (not it terms of linking) and invisibly).
The question is, whether this is in violation of the GPL, when the end-user provided library loaded in my GPL program is not GPL-compatible (proprietary or closed source, for instance).
Notice that, the GPL program Audacity allows the same functionality - it can for instance load any VST-plugin, that may or may not be proprietary. I can even, through creative audio system routing, simulate the exact situation using a project in Audacity that can be routed through something like Logic, emulating the wrapped plug-in situation.
I guess the question can be boiled down to: Can GPL hosts support loading of optionally provided non-GPL plug-ins in this specific situation, where the GPL host imitates the non-GPL plug-in in what effectively seems like one plug-in?