Is there a way to set a non-rectangular die area for floorplanning and placement? For example, if you wanted to fit two L-shaped macros together?

There isn't a way to define a non-rectangular die area explicitly though, but you can emulate that behaviour with rectangles and obstructions.

Okay, so as long as the overlapping rectangular cells have keep-out areas which prevent them from placing/routing in the overlapping areas, it should work?

Yup

Thank you, that information is very helpful. I'm trying to fit some 'T' and '+'-shaped routing macros between larger rectangular blocks arranged in a grid, so it sounds like that approach should work.