I have escaped the '#' character in the usual way, using a backslash, as '\#' in a btex...etex picture as part of a MetaPost figure created within a ConTeXt document, but the escaped character still causes the ConTeXt processing to fail.
Here is an example...
\pdfminorversion=5
\definepapersize[LG1][width=4.25in,height=6.875in]
\definepapersize[LP1][width=4.25in,height=6.875in]
\setuppapersize[LG1][LP1]
\definelayout[frontmatter]
[topspace=0.222222222222in,header=0.555555555556in,height=6.43055555556in,footer=0.694444444444in,
backspace=1.05555555556in,width=2.86111111111in,location=middle,
leftmargindistance=0.222222222222in,rightmargindistance=0.222222222222in,
leftmargin=0.833333333333in,rightmargin=0.111111111111in]
\definefont[TitleFont][Serif sa 3.5]
\definecolor[TransparentWhite][b=1,t=0,a=1]
\def \FrontMatterTitle {
\startreusableMPgraphic{frontmattertitle:\overlaywidth:\overlayheight}
matter_paper_width := 4.25in;
matter_paper_height := 6.875in;
front_left_margin := 0.388888888889in;
top_margin := 0.347222222222in;
draw (0, 0)--(matter_paper_width, matter_paper_height)
withpen pencircle scaled 1bp withcolor white;
picture titleText;
% == HERE IS THE SOURCE OF THE PROBLEM ==
titleText := btex {\vbox{\hsize 2.72916666667in \baselineskip 38.0pt
\setupalign[flushleft,hyphenated,verytolerant]
\TitleFont THE BEGINNINGS OF \#BOOM}} etex;
labeloffset := 0bp;
label.lrt(titleText, (front_left_margin,
matter_paper_height - top_margin));
\stopreusableMPgraphic
\reuseMPgraphic{frontmattertitle:\overlaywidth:\overlayheight}
}
\defineoverlay[frontmattertitle][\FrontMatterTitle]
\starttext
\setuplayout[frontmatter]
{\TransparentWhite\title{...}}
\setupbackgrounds[page][background={{frontmattertitle}},state=repeat]
{\TransparentWhite\midaligned{...}}
\stoptext