Sorry about taking so long to react.
In the days of yore when MathML support in Mozilla for Windows and Linux was designed (there was no Mac OS X back then and rbs didn’t have a Mac anyway), it was thought that Mozilla can’t deal with astral characters. I am not sure if this really was the case back then, but it is not the case now. The string API says “UCS2”, but the i18n guys have for quite some time treated the strings as UTF-16. This hasn’t been well communicated to all developers using the API, however.
Anyway, in order to stay on the BMP, which is no longer necessary, the DTD in Mozilla’s catalog fakes the astral math characters by mapping them to PUA characters instead of the astral characters the real DTD would map them to. The Win32 and X11 gfx special case these PUA characters and give them special treatment using legacy (non-Unicode) math fonts. The Mac gfx seems to be broken, as usual. Also, requiring legacy fonts on Mac OS X would be a bad idea, because ATSUI doesn’t know the fonts lie.
With the pure UTF-8 approach using a real astral characters, the UTF-8 astral characters are properly converted into UTF-16 surrogate pairs and travel through the application without getting caught in any special MathML-related code. When they reach gfx on the Mac, they go down the code path intended for astral CJK characters. (Comparable code path exists on Windows.) The surrogate pairs make it to the system API intact and get rendered. It seems that there are problems with the baseline. As usual, the Mac gfx is broken in some way even when it is partway right.
In fact, I think the Mac gfx is so broken in so many ways that it should be abandoned and reimplemented using ATSUI and Quartz. The sad part is that the gfx API was designed for GDI, raw X11 and QuickDraw. I think it would really benefit from an overhaul done with Quartz/ATSUI, GDI+/Uniscribe and libart/Pango in mind. However, to make that happen the drivers who get paid for working on Mozilla would need to be committed to such a major cross-platform change. Still, even with the current gfx API, I think the Mac gfx needs a rewrite. The piecemeal fixes just don’t cut it.
I would proceed here by installing a font that is properly encoded to provide glyphs for the astral chars and seeing if that works on Windows. It would certainly be interesting if it didn’t, because CJK astral characters work on both Mac and Windows provided a font has the right glyphs.
I finally got around to filing a bug about this.