How To Write Non-Unicode Symbol?

What Unicode construct has the best semantics for representing an
oddball non-Unicode character?  If I am writing a novel in which
there is a currency symbol, the Quatloo, represented by a glyph
from a custom font (say, a Q with three lines through it), how should
I write it?  <phrase role="quatloo">Q</phrase>?

It's inconsequential, since I can do whatever I please--this document
will not be exported to anyone, after all--but I'd like to get it 'right'
in some way.  So, it's just a matter of my curiosity here.