> 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.