There have been a number of occasions when I was not navigating to any specific place, just a general area "somewhere around the centre".

Who runs the heuristics and makes assumptions depends on the agreed interface, does it not? You can have a dumb client and a smart server or the other way around. I am mostly leaning towards a dumb server and rely on the client to formulate the query properly. It makes for a more flexible design as the server can be more easily reused in different clients, but it's up to you.

Regarding the "geo:x.xxx,y.yyy" notation, it seems to work after all but only for positive numbers. "geo:55,1" works fine, "geo:55,-1" gives "no results". It must be a limitation on the input field because all destinations I've tried so far have been on the western hemisphere with negative longitudes and they all worked fine, so the backend must be alright.

EDIT: I see we cross-posted. Kewl. Thanks for answering.