As a frustrated architect/designer I find DF to be an excellent exercise for efficient layouts. So the lack of design tools in DF is a bit of a hindrance in experimentation.
So I have a couple ideas:
My suggestion would be to use something that is similar to the Soldier Patrol path creator, except use it to designate areas for digging or building walls. Drawing a 45° hallway would be trivial rather than a carpal tunnel nightmare. Setting up hexagonal chambers would be a minor chore.
Using vector information, you could designate an area or just the outline of an area, perhaps change the thickness of the outline, like you would with strokes in some Adobe layout programs. You may even be able to select a circle and its radius similar to how you create floors or farm plots.
How feasible this may be is unknown to me, but it seems that there is already some sort of vector tool in DF to create the Patrol Path.
On another related note:
Perhaps, once you create a shape (even using the tools that are currently in DF), you can use it more than once, a sort of stamping out the shape (imagine if you only had to make one 10 tile wide wall designation and mark multiple areas with it, or draw out a bedroom shape with the dig tool and stamp out 10 or so with the same shape, rotate it, flip it).
Anyway, I apologize if this was already covered, but I didn't see anything in multiple searches. As a designer at heart, I really love DF for its integration of design and concern for efficiency with aspects of resource management and battle, not to mention the surprises that the game seems to constantly generate. I feel like fortress design is a HUGE part of DF (for me, at least) and often I wish I had more layout tool options.