After sttudying the sources and a bunch of trial and errors I can come up with the following solution which works for a two solumns set!
Great. This works. Now for something different. While learning ConTeXt, I find that often the syntax escapes me. Take for instance:
\definecolumnset[example][n=2]
\define[1]\MargText{% \ifodd\mofcolumns{% \inleft{#1}} \else {\inright{#1}}\fi}
\starttext \startcolumnset[example]
\MargText{Here left of column}
\input tufte
\column[local]
What does [local] mean here? According the the manual one should say \column and I can't find the explanation of what local means or what you are doing here.
\input tufte
\MargText{\ifodd\mofcolumns{Hello!, still left of column}\else {Hello, now right of column}\fi}
\stopcolumnset \stoptext
I believe that Hans will make a generic solution...
G -- "To be or not to be, that is the question" -- Parmenides