I just had a facepalm moment
Right under my magma cistern there was a single tile of gold ore, which I want.
Well, no problem, I'll just selectively cave in the place around it so that my magma forges keep going, and the extra magma can be drained into the ocean. And look, here's a tile I can drop in the path of the magma, so it will all go the right way.
I then start channelling to get the tile attached to just a support with a lever.
I then look down to check on the gold, turns out, I channelled the gold I wanted to get the tile free, and there's now 2 levels of 7/7 magma on top of the gold ore.
*Facepalm*
(Now in ASCII form, spoilered)
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