Got it! No external commands, no backquotes, no magic functions:
${0+:} echo This is es.
or, conversely
${0+:} FN={}
$FN echo This is sh.
The magic sh variable expansion simply looks like a contorted variable
name to es. Of course, we're assuming that no one's using the
contorted variable name, but anyone who does that deserves what they
get. (-: