That should work
for Ibn Saeed, but it's not a complete solution. For example it does
not work if one wants to write "page x of y" instead, with "page" and
"y" written in Arabic/Persian. Then the text of the header is partly
LTR and part RTL.
This is still simple and I can use
my hacky solution and mix \lefttoright and \righttoleft to get it
right. But is it possible to extend the \setupdirections[bidi=on] to
apply to parts other than the main text? I just noticed this does not
apply to footnotes either. Having numbers in the TRL footnote text
produces incorrect typesetting.