I'm not entirely sure. Now that I think about it, fine grained undo is really a different concept from global state restoration, and is commonly only done for variables that the programmer a) knows about and b) has in mind the place he would like to restore to.
This means that finer grained undo would be more accurately implemented with save and restore functions, as opposed to just an undo function.
The global undo should work the same was as previously stated, more like a reset, and going all the way back to the start point, which may or may not be the beginning of the w/undo block.
Maybe a better system would be two pairs of save and restore functions, one that works on individual symbols, and the other that redefines '= to store the old value in a table if it didn't already exist, so that reset could restore it.