tikz matrix of matrices

[ptangr]

I would like to create displays of the form
 
Block_1 Block_2 Block_3
Block_4 Block_5 Block_6
 
where 1) the Blocks 1,2,4,5 have aligned rows and columns
2) Blocks 3 and 4 should display text that spans
the full width of the corresponding Blocks 1,2 and 4,5
to their left
 
Q: How could one build this?
There does not appear to be a concept of spanning
rows and columns for Tikz matrices.
 
Note that the vertical length of Block_3 might exceed the
vertical length of Block_1,2  
 
Q: Is there a way of adding matrix style left and right braces
around each of the blocks?

[ollywoodman]

I have also been looking for a way to do row/column spanning in matrices. It seems like a pretty big omission if it isn't there! The best idea I had was to embed one matrix inside another (which would effectively allow you to do spanning), but this isn't allowed either!