That’s a clever move, but if Blender never considered adopting OpenFX, why would they support openmesheffect? You would have to be more convincing (or maintain your own blender branch)
About the non-GPL plugins used by a GPL app: I would say that as long as the app is fully functional without the plugin, and the plugin is distributed separately, then the plugin is not a component of the app.
People in the Natron community do not think that people infringe the GPL by using non-GPL (including commercial) plugins, because Natron can be used without these commercial plugins. But, as the GPL FAQ states, “This is a legal question, which ultimately judges will decide” https://www.gnu.org/licenses/gpl-faq.en.html#MereAggregation . Note that this sentence has been in the GPL FAQ forever (at least 18 years, see https://web.archive.org/web/20010804115858/http://www.gnu.org/licenses/gpl-faq.html ) and judges never had to decide!
Whatever Blender or GPL afficionados may claim, this is just common sense. And this is just the same as with Linux proprietary kernel modules. Linux is still GPL but is capable of loading proprietary modules. You can read Linus’ thoughts about that: http://linuxmafia.com/faq/Kernel/proprietary-kernel-modules.html
If you really want to protect yourself against this situation, I would say that the OpenFX API should be rather easy to serialize through sockets, at the expense of performance. In fact, this is a solution that I looked into when I first wanted to make a GMIC plugin (GMIC has a GPL-like licence, and Natron was not GPL at that time, and we even kept the possibility of a Commercial licence for Natron until last year).
My goal was to make an “isolation layer” that would convert API calls to socket read/write on the OFX host, and on the other end a server would be in charge of loading the plugins and executing the stuff. Because OpenFX uses “handles” - (with a few exceptions such as
kOfxPropHostOSHandle which is really a pointer), this should still be possible. But, in my opinion, this is a waste of time.
You can thank Bruno Nicoletti (founder of The Foundry) for such a great API.