Firefighter
I have a lot of faith in IF's, but the scenario you're describing would really be pushing it for me.
I believe it could work, but I don't know that you'll get to the point of just letting your pup out the door to do his business without supervision. As you stated though, a barrier type fence is in the future, so it would be a short duration, and there is nothing wrong with using an IF and a barrier fence in cinjunction with one another.
The best fence, in my opinion, is by Invisible Fence. They also do the installation,and from what I've been told help with the proper introduction of both the dog and owner to the fence.
I use a fence by Innotek, but my location is somewhat rural, and the "girlz" have been on the the system for going on 3 years without anyy incidences.