earthdistance

I develope a project openbarter that needs to match objects based on a
maximum distance between their positions on earth. I saw that the
documentation of the extension earthdistance was interesting, but the
promise was not in the code. It would be nice to have these functions
available independently of sophisticated geographic systems. There is a
circle object for flat two dimensional space, but earth deals with
spherical caps. It would not be exact but enough to suppose that earth is a
sphere and that all dimensions latitude, longitude and distance are in
radian.
What would need to be done to adapt the circle type to a new type
'spherical cap' that would allow simple geographic indexing?

--
Olivier Chaussavoine