You can get of the warning by changing the alignment for the \margindata
entries.
\setupmarginframed
[inmargin]
[align={flushright,broad}]
\starttext
Overfull\inmargin{abc}
\stoptext
You can produce the same message with the following example.
\starttext
\vbox
This minimal example:
\starttext
\currentdate-\currenttime\quad\ConTeXt-version=\contextversion\blank
Testing \ConTeXt macro\ \tex{inmargin}
\blank
Some text to fill the body of the text.\par
\inmargin{\hbox{abc}}
Some text to fill the body of the text\par
\stoptext
produces an underfull hbox:
