> It's a single tex run so you can do:
> \starttext
> \newbox\MyMpBox \setbox\MyMpBox\hbox{foo}
> \startMPcode
>    draw textext("\copy\MyMpBox") ;
> \stopMPcode
> \startMPcode
>    draw textext("\copy\MyMpBox") rotated 45;
> \stopMPcode
> \stoptext

This is not much different from:

\starttext

\startMPinclusions

    string t;

    t := "foo";

\stopMPinclusions

\startMPcode

    draw textext(t);

\stopMPcode

\startMPcode

    draw textext(t) rotated 45;

\stopMPcode

\stoptext


but the problem was not with passing just text, but with more complex
images that may or may not contain text, without specifically addressing
text after image is defined.