That's an interesting idea, but there are few quirks to it. Be curious to hear your response to the following comments.

First you'll need to pick a fuse/polyswitch that shuts off at ~half your target current draw. Eg if you want your protection circuit to kick in at 5A draw and you have a polyswitch/fuse in parallel then some ratio (directly proportional to the resistance of the polyswitch and fuse) of current will flow through the fuse and some through the polyswitch. Obviously if they have equal resistances then if 2.5A flows through each device you'll have a total draw of 5A.

I'm assuming that they have near equal resistances when I say "~half" above. Clearly that assumption isn't good. The resistance of the polyswitch increases (possibly very slowly) with temperature. This raises some interesting issues...

Ok, let's say you've solved that problem. Now your device has a short and starts trying to pull >5A. Ok fine, but now your fuse blows because more than ~2.5A flows through it and now ALL the current will try to flow though your polyswitch so it too will reset. So what's the point of your polyswitch again? Now the whole thing will stop working and you'll get the call "it doesn't work." Ok, open it up and you'll see the "indicator" of a blown fuse. But you'd have seen that anyway if you had just used a fuse in the first place without the polyswitch.... So what does the parallel polyswitch do for you?

D