@craigappl, regarding the question about connecting from the registrationapp to OpenEMPI, there was an effort to enable this (optionally) via a Google Summer of Code project that @darius mentored 2 years ago.
So there may already be a bunch of work done on this that you will be able to leverage, if it matches your planned workflow.
Googling “openmrs” “registration” “mpi” brings up several hits. like this and this and this.
Awesome that the team plans to work on this in the coming months! Mike