yah, that seems to be the only limitation to her god-like ability of reversing events. if the amount of reiatsu or mass being handled is really great (re: healing ichigo in hueco mundo, and healing the gate guardian at SS), she has a lot of trouble doing it.
personally, i doubt orihime will be able to reject the hougyoku - first of all, isn't the hougyoku supposed to be an incredibly powerful item ? erasing its existence should be outside orihime's limits. second of all, aizen isn't an idiot.. i'm sure he'd be able to stop it.