What you ask for is pretty simple, but I don't think anyone will bother to do it anytime soon.
As I recall Netscape used to save its bookmarks in html format (you can even set this as your home page!) and I just checked and Mozilla still does this (in your .mozilla directory). You can edit this file externally and it will load no problem. Konqueror uses xml and is readily altered as well (in .kde/share/apps/konqueror). I imagine that all of the other major browsers do nothing particularly special when storing bookmarks so writing scripts to transfer them from one format to another is not difficult... but why would someone bother?
I think that MOST people use bookmarks to flag the five or six cites they use constantly (webmail, etc) and rely on a search engine to get them anywhere else. If you have a LARGE collection of links which you wish to keep, I would recommend creating a local set of html pages to store them all.
That's my 2¢