Ahh yes. Working on that type of thing too, as of speak.
Tough stuff.
Maybe this can be of help as well, regarding getting the face rotation in space.
http://sketchucation.com/forums/viewtopic.php?f=180&t=33862&hilit=plane+on+plane
I'm doing something similar to Cleverbeans solution currently to find a face edges rotation in space. However it is difficult to exactly tell what a face have for "intentional" rotation depending on several matters. Where the edges are in relation to face and so on.
Scewed, tapered, N-gons and quadrilaterals faces etc that may not be perfectly square
need a lot of testing if you are going to align geometry on to them.
Its quite frustrating business. As soon as something gets fixed some new problem arise and need further testing. There are of course mathematical formulas to calculate all this, but honestly. If one has that kind of knowledge one shouldent be sitting and programming Sketchup plugins for free.
It's damned rocket science as I see it.😄