Well, this isn't technically multitile, but what if the megabeast had an overlay or background tile on each of the spaces next to it. It wouldn't actually take up those tiles, but it could reach into tiles outside of them, similar to how a bronze colossus has a 1 tile reach.
A monster could be displayed as
_
/ \
|O|
\_/
But could have an adventurer run up into reach
_
/ \
|O|
\_D
D=dwarf
This wouldn't make some of the ideas like a monster being blocked by a small corridor possible, but it would look better and be able to use some of the code already present.
Also, entering the tiles that are inside the megabeast's area could have a chance to fail and be knock you backwards. This would make having an army of peasants less powerful against it, as they would fail the skill checks more often and be knocked back before they can do any damage while a skilled fighter will be able to avoid being smacked backwards more easily.
Walls nearby could also be knocked down, making it a good idea to take out the monster before it can get to your fortress and start running through the hallways smashing down your walls. Another alternative would be to have a lower speed when trying to crawl through 1 tile spaces.
Since only one megabeast would ever be on screen at a time, this would probably be more feasible than having to path a multitile creature, as it could be limited to a finite number of surrounding tile checks instead of pathing. Depending on how toady implements the code, it could be like having 20 goblins on the screen, but while only having do the expensive pathing for one of them.