When everyone is at a safe distance yank on the rope really hard, pulling the support out, causing the section of wall to collapse. No explosives needed.
Hmmm... While I was of the "Levers are Dwarfy, fire can't be handled, explosives aren't Toady's bag" opinion, and thus thinking levers, I like this new use for ropes. Don't know how the mechanics would work, but given how a current well of mine uses a single rope to stretch ~30 Z-levels (and would doubtless span 300, if I had such a set-up[1]), I could see a key support being pulled away by a rope from a location (with a straight-line pathing[2], unless you assume/require 'pulley' mechanisms) being a nice compromise between the automagical lever idea and bringing the sons of the soil back down to earth... Gives them a little more leeway so they don't have to run quite so far to safety when the breakthrough is made.
Also gives the possibility of demolishing more than one segment of contiguous wall at a time, which is often the problem with either dig'n'run or fortify'n'run, when you'd ideally want more width for the liquid to flow through. Either multi-tie the rope (like a single lever to multiple floodgates/doors/bridges/supports/etc), or treat a fully connected (orthagonals only) section as a single entity, so that when a single section is primed to be deconstructed either by rope or mythilever it all collapses. Perhaps with a 'domino delay', if you want it more 'life-like'.
[1] Yet at the same time keeps creatures tied to within one tile of a location without any apparent spare loops trailing all over the place
[2] Not sure how diagonals in X,Y and/or Z would work. Well, not with representation.