Thanks to maciejr we know now what the remaining problem was with bug 7954:
My unicode symbol fallback works fine, the problem was that a font named
"Symbol" is available on OS X, but it does not use the font-specific encoding
we expect: Almost all glyphs are at their unicode code point.
Therefore the bug is fixed by re-enabling the unicode workaround and blocking
the Symbol font on OS X.
c1ac09e11f