I checked Gaspy's BOC - and it is an upkeep(!) spell. I was originally going to use Feora's firewall as an example, but that is upkeep as well. Looks like Privateer is doing a bang-up job of keeping the rules loopholes to a minimum - unlike some other companies I know.
This won't come up often, but what happens if you are playing a team game and you - for sake of arguement - throw up an imprision, and then Feora throws a firewall on the guys stuck in there. There doesn't seem to be any ruling against stacking AOE effects, since you should be able to throw a firewall onto those pesky smoked trenchers, as long as it is within the spell range (I believe the range of firewall is CASTER). Bwahahah - burn baby burn!