Spannable cells is an interesting one. What is meant by spannable cells is the ability for a cell to flow over into another and not have the vertical cell border drawn. After this hurdle is overcome, there is absolutely no reason why somebody (not me, though - I'm to busy making the changes) can't write a package that will emulate Exc*l and an end to the pathetic existence of spreadsheets on Unix.
In essence, it can be done already by obtaining the widget ID of the XmTextField and setting resizeWidth to True. Problem is, though, after committing the changes to a cell, it pops back to it's usual size.
The changes made to the drawing of the row shadow should also facilitate the implementation as it is now drawn on a per cell basis.
Again, there will be significant changes to the code but, provided as a resource on an as need basis, there shouldn't be a significant change on performance nor memory consumption.