Thank you Andrew
People, I got the formal agreement from Andrew Fedoniouk, Blocknote for Net's author, to reuse the UI metaphor he implemented for table column/row resizing. I already started working on it, it's going to be a marvelous feature for Nvu. Thank you so much Andrew.
Update: here is a screenshot. The horizontal and vertical rulers show the positions and sizes of the rows and columns of the table. Everything can be manipulated with the mouse. It's really neat, and it's MUCH simpler to implement in Gecko's model than the common click-on-borders model.