Defining very font family specific font "fail overs" via faked FAST/FV

I'll try to explain this the best I can, here goes:

One if the questions asked after Bart's CSS One Year Later session touched on creating a new FV record that referenced multiple FASTs. The example of why someone would do this focused on combining Times with a Times version that included cyrillic glyphs.

The thought here is that if one foundry's version of Times didn't contain a specific glyph, I could attempt to fail over to another foundry's version - and hopefully minimize any visual differences.

An example would be that if Times STD didn't have a specific bullet type, but Times PSMT did, XPP would take the bullet from Times PSMT instead of failing over to the more general glyphs contained in the secondary FAST entry.

Any chance someone involved in the conversation could provide a bit more info (our at least where to look in the doc to find out more about this)?

Kind regards
-Thomas