Throwing 2.0 into our dune-projectdid indeed neatly solve the problem. Thanks @jeremiedimino!
As suggested, opened an issue here to talk about this feature further:
As for this part:
Sensible, although you could imagine the (hypothetical!) external install program/command-line that you’ve specified to install files can escalate its privileges as needed, such as with sudo. Then you could build and install in one go.