I have created an issue for this, but not much movement nor any comments so far. For MPI I do need boost 1.65.0 OR 1.65.1. My preference is for 1.65.1; note that BH on CRAN is at 1.65.0.
The MPI stuff can be setup to run if one tells the Stan build system to use a locally installed boost 1.65.1 in place of the current boost 1.64.0. Note that in boost 1.64.0 the MPI library is BROKEN (for the record, see here: https://svn.boost.org/trac10/ticket/12723).
I know how much pain it was to upgrade to 1.64… however is there any opinion on moving to 1.65?