How to implement a SP-GiST index as a extension module?

Hi there!

I'm looking at implementing a custom indexing scheme, and I've been
having trouble understanding the proper approach.

Basically, I need a BK tree, which is a tree-structure useful for
indexing arbitrary discrete metric-spaces (in my case, I'm interested in
indexing across the hamming edit-distance of perceptual hashes, for
fuzzy image searching). I'm /pretty/ sure a SP-GiST index is the correct
index type, as my tree is intrinsically unbalanced.

I have a functional stand-alone implementation of a BK-Tree, and it
works very well, but the complexity of managing what is basically a
external index for my database has reached the point where it's
significantly problematic, and it seems to be it should be moved into
the database.

Anyways, looking at the contents of postgres/src/backend/access/spgist,
it looks pretty straightforward in terms of the actual C implementation,
but I'm stuck understanding how to "install" a custom SP-GiST
implementation. There are several GiST indexing implementations in the
contrib directory, but no examples for how I'd go about implementing a
loadable SP-GiST index.

Basically, my questions are:

* Is it possible to implement a SP-GiST indexing scheme as a loadable
module?
o If so, how?
o And is there an example I can base my implementation off of?

I'm relatively comfortable with C (much moreso with C++), but I haven't
spent a lot of time looking at the postgresql codebase.  I don't think I
could start from a empty folder and make a properly-implemented module
in any reasonable period of time, so if I have a working example for
some sort of index that uses the same interfaces that would really help
a lot.

Thanks!
Connor