Working on typescripts for Y&Y's LucidaBright (which are type1 fonts) for MKIV (hoping to maintain compatibilty with MKII), the following code produces all math in bold, even the first x+y.
  \usetypescript[luc]\setupbodyfont[luci]\starttext $x+y$ -- $\bfmath x+y$ \stoptext

I studied the new manual chapter on typescripts as well as have worked in analogy with existent typescripts in the current ConTeXt base. I certainly am missing an important clue here.

The relevant portions of the typescript are:

\starttypescriptcollection[luci]

\starttypescript [luci]
  \definetypeface [luci][mm][math][lucibr][default]
  \definetypeface [luci][mm][bfmath][lucibr][default] % removal prints nonbold math only
\stoptypescript
...
% typescripts for text in serif, sans etc. omitted but contain:
  \definefontsynonym [LucidaBright][lbr.tfm][encoding=\typescriptthree]
  \definefontsynonym [LucidaBright-Demi][lbd.tfm][encoding=\typescriptthree]
...
\starttypescript [math] [lucibr] [name]
  \usemathcollection [lucibr]
  \definefontsynonym [MathRoman][LucidaBright] ... etc.
  \definebodyfont [default] [mm] [mr=MathRoman mo 1, ex=MathExtension mo 1, ... etc.]
  .. math font tfm definitions
\stoptypescript
\starttypescript [bfmath] [lucibr] [name]
  \usemathcollection [lucibr]
  \definefontsynonym [MathRomanBold] [LucidaBright-Demi] ... etc.
  \definebodyfont [default] [mm] [mrbf=MathRomanBold mo 1, exbf=MathExtensionBold mo 1, ... etc.]
  .. bold font tfm definitions
\stoptypescript

\stoptypescriptcollection

Hans van der Meer