Allow me to ask a further question that may be easier for someone to answer now that I think I might get it myself.
It looks as though qtree-level exports merely allow you to make access unique to the underlying qtrees within a volume, but access still needs to be allowed at the volume level for all rules defined at the qtree-level.
So access to underlying qtrees can be more restrictive than access at the volume level. But in the case where you want to give more access at the qtree-level than you do the volume-level...it's not possible due to the way junction paths function requiring the hierarchical access. Is this correct?