Tim, it should be possible that the main .exe (or binary on linux/osx) to export functions, and they can be found back through the flutter_engine.dll/.so, right? (At least on windows it's possible for the exe to __declspec(dllexport) things, not sure on Linux).
But even if not, if there is a way to (given might be slow and tedious) to compile what would've been flutter_engine.dll statically linked to your main (host) app, and maybe somewhat hinting here is how to find the "GetProcAddr/dlsym/etc." functions - if one is to look for them.
Further on, the generated from dart .so file can be embded (as resource? or slapped at the end of the binary/.exe?) + slapping all other resource - resulting in single app :)
I know it's possible, just not so easy to support it (I guess). It's similar to have java/python can be packed into one executable with all C/C++ ffi functions they call (at cost of some build time and effort to achieve so).
Then you'll have killer platform for video conferencing (zoom, bluejeans, etc.), messaging (slack, etc.) - and just single binary to deploy.
(Oh, on top of that slap a web server into it, and make it that if called with specific command-line option, now it acts as web-server, and the app is visible through the web).