Nope, only ~10 Annoscales in each dwg.
After spending hours on this yesterday, I went back to the dwg in which I unloaded all xrefs. It opened fine & relatively quickly. The only things really missing were the Sections derived from the Xref'ed corridor. I reloaded the Corridor.dwg, and it did so without crashing, however none of the previously sampled corridor sections were shown in the SectionViews or in the Sections listing for the SLG's. I began editing each SLG, adding more sources to include the Corridor, saving after each addition fully expecting one of them to cause a massive KaBoom!. After re-adding those sections to all of the Alignments, the drawing was still behaving well, saving quickly, no apparent hiccups at all. I made some additional edits, found an area at one of the street intersections that needed some massaging (which I knew about and what started me on this journey to be able to use these sections), saved, closed, opened the corridor.dwg, made some edits which should've cleaned up that intersection, saved, closed, opened the Xsections.dwg to verify the cleanup worked, and.....there's that KaBoom! Back to where I was at the beginning of the day, with a dwg that will no longer open in C3D.
I'm now going to save that drawing 9 or 10 times and, using plain AutoCAD, edit each one to only have 1 or 2 alignments/xsections. In my initial test of 2 drawings, this allows the drawings to open in C3D. So this leads me to think that there is some kind of limit to the number of SLG's, or SVG's in one drawing. Or there's a conflict with one, or more, of the sample lines. I'm at a loss as to why this is occurring, but it's time to move one and get the job out the door.
Thanks for the suggestions and for listening while I rambled on.