@menno it sounds like the preCICE library is indeed something for you.
That’s exactly what preCICE expects.
preCICE is primarily designed with surface coupling in mind, but some users have also successfully used preCICE for volume coupling. Read more in Can preCICE be used for volume coupling?.
I am not sure what or how exactly you would like to couple inside the domain (for overset grids), but maybe it also helps to read FSI simulations with overset grid in OpenFOAM (FSI where the interface is part of an overset grid).
Does this answer your question? Feel free to add a figure decsribing your problem, to discuss more details.