I did a quick read up on mobile internet in the Philippines. Is the 3G connection that bad at the shop that you need to use WiFi? I suspect that this is reason the question came up in the first place, but I thought I make sure regardless. In most of Europe as well as north America most people would probably assume that customers have a 3G/4G data plan if they pay with bitcoin and dont bother at all.

More towards the actual topic, I would not try to block all but bitcoin on the port layer, but go to the application layer (7). If you have a router that can be run with open firmware solutions like tomato or OpenWRT it should be possible to create a bitcoin only WiFi. This might get tricky to setup though as I doubt there are ready to use filters like there are for other p2p traffic like torrents.

Maybe you can find a "good enough" solution by blocking HTTP(S) and mail as well as other commonly used services that generate a lot of traffic without actually having an airtight solution.