I figured out my problem... rather than the jboss-ql from before, I tried this & it works... Lovely!
Still getting the hang of this ejbql stuff.
FROM CategoryEJB AS c, IN (c.positions) AS p
WHERE c.id = ?1
ORDER BY p.position DESC
The problem is with the parameter type. JBossQL is looking for a parameter of the correct EntityType, which means either a local or remote interface.