but it would be nice to have a way to control it.

You can already (indirectly) control it — at least in the direction of forcing the use of the shell.  Just add a dummy ";" to the command (which should always be possible without changing the semantics).  Or simply call sh -c "..." yourself.

(And be sure to add a comment regarding the ";", or else the next maintainer might remove that seemingly superfluous character again... :)