Jim -
I'm sure you have either found your way through this problem, or given up on it by now, but there is at least one of us out here who shares your pain. COMcheck is somewhat specialized, even in the US, hence the lackluster response to your question.
I think you have nailed the source of the problem. Any old XML file will not do. A few years ago, I was able to get a workable starting place out of AC into COMcheck using a gbXML intermediate file I made using a 3rd party (Encina) translator. Encina is gone, but Cadimage picked up the translator. Jared Banks (Shoegnome) wrote about it Feb. 2015. Josh Osborne @ Cadimage posted a comment there indicating that the translator was then current for use through AC 18 or 19.
All that is only history for many now that AC20 has gbXML export built in.
It's hard to find, inside a new Energy Model Review Palette, but the Help files will steer you to it. It looks pretty good, but I'm just now about to start my first AC20 project, and have not tried it.
In the mean time (AC13-18 ), I have found it simpler to use AC to take off areas, and use COMcheck for the rest. COM√ allows a lot of aggregation of areas of wall types and openings. Unless you set up a set of rules for the export and model quite carefully and specifically for COM√ export to gbXML and into COM√, you will get a very long COM√ report that requires a lot of tweeking.
If you're running COM√ a lot, breaking floors, walls, and roofs into envelope and non-envelope pieces (e. g., to exclude eave and gable overhangs from counting as envelope elements) - and at least building a schedule for building envelope elements, their construction, orientation and areas of like walls, roofs, and floors - might be worthwhile, even if automating the entire process isn't.
