Any time, phatt !
I figuered Frank's work might be helpful or at the very least, interesting.
You're right about what a speaker sim actually does, it's "just a filter" and I wish there was a simple (stomp)box with a filter inside that was modelled after a really good sounding example of one of those digital cab impulses.
Taking the graphs of a speaker as a starting point is one thing. But these graphs never show how the speaker actually sounds in an enclosure with one or three compadres and different microphones with different placements also have their impact on the sound.
I wish I had the knowledge to come with such a box.