Pics or it didn't happen?
I said READY the cake--I'm preparing, but haven't done it yet.
oh I know, I know - I'm just saying, when you DO it, post pics or video for maximum awesome and cakular awards.
Also, I build walls around bridges all the time - prevents dodge-offs and there's not even a need to build+remove anything. Just build it from the outside-in:
1234321
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1234321
Numbers are the orders in which you build the walls.
Anyway, Grumbledwarf, if I understand your design correctly, keep in mind that the support only supports things directly above them (unlike a constructed wall). However, I've also used designs like this:
+++++++++
I
+
The I is a support with a single floor tile above it (to do this, build a wall adjacent to the support and a ramp adjacent to that).
The solitary + is a constructed floor (NOT a wall, or it will hold up the floor above the support!), although it can actually be whatever shape you like. You could build a 10x10 set of floors and drop them all at once.
Then, to activate it, hook up a lever (or just deconstruct the support for maximum suicide). The support will go away, the floor it was holding up will crash through the floor it was standing on, and one tick later the OTHER floor will fall, giving a nice 1-2 punch (or a single floor followed by a huge piece of landscape crashing down, your choice). I've used this to destroy migrant waves; just lock them out of your fort, have them build this over a volcano with the support on the rim-side of the walkway, put a meeting zone on the walkway, remove the support, bye-bye migrants.
The awesome, though, is that cave-ins destroy any buildings under them, including bridges. You know how you can't bridge-drop or atomsmash forgotten beasts or demons? Yes you can. Just drop a single constructed floor anywhere on the bridge and KABLOOIE. I'm pretty sure that the thing on the bridge (or army, depending) will fall straight down too, with no chance of being flung to safety. So you could make the only entrance to your fort be a string of lots of 10x10 bridges, each of which has a single constructed floor hanging over it; whenever you want, just make them all fall down and the WHOLE bridge goes, with everything on it, down a 50-z drop into magma or whatever. Bit of a chore rebuilding the bridges after every siege, but whatever.