I've added a custom font and defined symbols from it:

%%%%%%%%%%%%%%%%%%%%%%%%%
\loadmapfile[texnansi-glambats.map]
\definefontsynonym
[GlamoconRetrobats][texnansi-glambats][encoding=texnansi]
\def\GlamoconRetrobatsSym#1{\getglyph{GlamoconRetrobats}{\char#1}}
\startsymbolset[Glamocon Retrobats]
\definesymbol[GuruBullet][\GlamoconRetrobatsSym{121}]
% other symbols
\stopsymbolset
\starttext
\symbol[Glamocon Retrobats][GuruBullet] 
\stoptext
%%%%%%%%%%%%%%%%%%%%%%%%%

This produces the right symbol. But I can't figure out how to use that
bullet in an itemized list. I don't think I can use \usesymbols because
in some lists I want the regular bullet, and in others I want the custom
bullet. Any tips would be appreciated.

-- 
Stuart Jansen <sjansen@gurulabs.com>
Guru Labs, L.C.