Not really an answer, just a few hints.

Depends which "OpenStudio" (OS) you're referring to. OpenStudio-HPXML relies on OS/E+ engines, and is used by a number of frontend applications including BEopt (see Projects in the previous link). A lot of effort indeed seems to be invested in BPI 2400 compliance (see here), yet can't state if it's 100% compliant ... other UMH users (very active in this area) definitely can. Search UMH posts for "HPXML" (e.g. here, here).

I don't believe other OS frontends, like the Application, can import HPXML and populate an OSM file, yet I see no issue using the Application to open/modify an HPXML-generated OSM file itself. One way street, though. And at that point, I suspect one would then be operating outside the compliance sandbox. Hope this helps.