I am interested in creating additional field types for WSS. Eg. when adding a
field to a document library, there are certain field types defined - text,
boolean, date, number, list etc. I'd like to create a completely custom list to
get values from a place outside WSS and display it with a link to an external
system.

I can see that there's a fldtype.xml defining field types, with fragments of JS
and HTML for the field's display, but does anyone know how much you can add to
that? Or tried something similar? I get the feeling it may be quite complicated
- the add column admin aspx page has tons of hard-coded values all over it
suggesting that the api/code is wearing a little thin in that area...