I'm sure that there's a workaround (or at least a dozen of other
ways), but I have no time (and my computer is being repaired again) to
inspect right now.
MKIV is both very powerful and fast, so you could probably also define
strings inside metapost run, something like
string mystrings[]; strings[1] := "ABC"; strings[2] = "DEF";
draw textext(strings[1]);
Untested.