I don't see how the point size will be somehow different in aspect ratio from the pixel size?
why point/pixel sizes at all? for years the way to specify a font size for web is in "em" units. 1.0em 0.8em etc..
If courier 12 is say 10pixels wide then it should be identical to courier -10 visually, since the aspect *should* be the same.
even a 12 point font can be different on the same platform depending on which DPI you launched your X server at, which toolkit you are using, etc..
Or am I misunderstanding what you mean by the "at least three dimensions to a font size"?
id say theres at least 4... platform, dpi, font, fontsize
The font issue is really a stumbling block for trying to make anything look constant accross platforms. The current PD documentation is a great example of this, where, depending on the platform, many help-patches open with overlapping messages/comments. :(
how about a scalable canvas with a client-side algo to make sure nothing overlaps? something like http://whats-your.name/pd/chokun.png but with less ugly colours?