One possibility: it may be worth looking at what existing field types allow.
For example, if you have
{ set customer "Spender,B" }
{ ref customer }
the user sees
Hey, big spender!
but you can also insert
{ ref customer action="bought everything" }
and you'll still see the same result. But maybe there are limitation on the
characters allowed in the text after the ref name, the overall field length,
and so on. And of course this approach could easily be undermined. Depending
on what you are attempting, maybe you could implement something along the
lines of
{ ref customer action=myaction }
and insert a Word document variable (or perhaps document property) called
"myaction", IYSWIM. Also easily undermined, but everything is still
contained within the same .doc file.