[NTG-context] Chapter heading with rules

[jbf]

Have achieved a Chapter title with rules either side (see below - not an 
original invention!) but I want the chapter number to be part of 'My 
Chapter Title'; instead, the following MWE means it comes before the 
ChapterLeftRule. I know I can do it by actually putting the number 
before My Chapter title (e.g. 1 My Chapter Title), since the proper 
numbered version still appears in the TOC, but my preference would be to 
include a command in the \setuphead, if possible (so long as it still 
allows the numbered chapter to appear in the TOC of course). Possible?

\definefiller
   [ChapterRule]
   [alternative=rule,
    color=gray,
    height=0.8ex,
    depth=-0.6ex]

\definefiller
   [ChapterLeftRule]
   [ChapterRule]
   [left=\zerowidthnobreakspace,
    right=\quad]

\definefiller
   [ChapterRightRule]
   [ChapterRule]
   [left=\quad,
    right=\zerowidthnobreakspace]

\setuphead
   [chapter]
   [%align=middle,
    textstyle={\setcharactercasing[WORD]\ssa},
textcommand=\groupedcommand{\filler[ChapterLeftRule]}{\filler[ChapterRightRule]}]

\starttext

\chapter{My Chapter Title}

\stoptext

Julian