Or would it leave 1 stacks all throughout the chambers beneath the forges?
AIUI, 1-deep magma evaporates into thin air, much the same as water. If this is the case (I'm an "all or nothing", person, so 1-depth magma is usually only an issue for me at the forefront of a new route being flooded into) then as long as you can drain one spot of a magma-filled area, for long enough, any other 'column' of lava will redistribute itself around within the same area until it too finds itself drained (up, or down), and the process continues.
If you don't have an "infinite dumping ground" (or at least large enough to drain it all and let it settle into 1/7ths and less) I would personally try (for laughs, at the very least) a pump (or hatch) with a magma-sensitive pressure plate on the 'dumping' point that activates it only when there is no magma on that spot, to draw up/dump as much magma as available (the full 7/7ths of the one spot being drained) in the first instance, and with at least 7 squares of 'evaporation chamber' (but make it two or three times as many, to make sure) for this to level out into and dissipate. Immediately the dunmping starts it'll shut it off, but when it's gone from the plate-top (noting that it might still be 1/7 or even slightly more in the other spots, as well as a delay in reaction, hence the multiplication of the space you make available) more comes in and redistributes accordingly. It'd also be more efficient in its evaporation if it "spreads", rather than flows along a tunnel.
I'd probably try one of the following solutions (first for a pump from a channel, second for a lower-level cistern centred below a hatch, other variants are possible for same-level pump/floodgate/doorway control methods).
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I think that floodgates also "chomp" anything that is flowing through them when they close (as my concern is usually preserving liquid water on maps with too little available, I try to avoid this situation, as well as 1/7ths evaporation, and without checking the wiki I can't be sure this is the case for magma), so maybe this could be developed into a method of getting rid of excess fluids. I'm also not sure if you can Atom-Smash a flooded area free of its liquid (a 1x10 tunnel, or even a 10x10 cavern, with a bridge (suitably magma-proofed if necessary) lowering from the far end when a 1x1 alcove at one side detects the presence of the liquid), but something like that might be worth a try, if there's nothing on the Wiki or here that gives a definitive no!