I don't think it's possible to get that to work the way you want.

The way that the prepare/execute combo works is that the prepare parses the query and figures out the most efficient way to run it, then execute just plugs in a few constants to define the details. A subquery, however, isn't a constant, it's something else which has to be parsed and planned along with the main query, so I really, really doubt that it's possible to use one as a parameter.

Probably the best you're going to be able to do is move the prepare into the loop, change it to prepare_cached, and put the actual subquery into the statement you're preparing. You're going to have to do separate preparation for each subquery, but prepare_cached should at least prevent you from repeating your work when/if multiple rows have identical subqueries.