Javier Múgica
While computing gliph spacing by analyzing the glyph's shapes may be too involved, a good, easier solution could be that, in addition to the rectangle enclosing the glyph, another shape were known, not so complicated as a glyph's shape but not so crude as a rectangle.
This is roughly what I do with my (unfortunately not documented) "spacing by anchors" fontforge scripts at sortsmill.googlecode.com. I say "roughly" because I use only a very limited number of manually chosen horizontal cross-sections of a "virtual" surrounding shape, and because the samples are not stored in the font (there being no typesetting or word processing software that can use them) but used to compute class-based kerning tables. Also I have made no attempt to support mathematical typesetting. The idea, though, that typesetting software could use such an approach to space glyphs rapidly, on the fly, given the shape-approximations, has occurred to me. (With fontforge one could look at the spacing point placements of my fonts, for instance http://code.google.com/p/sortsmill/source/browse/lindenhill/LindenHill.sfd which I have been working on recently.)