Digging channels directly above semi-molten rock has strange effects.
The tile on the upper level (the one being channeled) turns into a "Magma Flow" tile. These tiles appear as dark red single- or double-tildes, even with SHOW_FLOW_AMOUNTS:YES. The display switches between the single- and double-tilde variants at some fixed interval, and the tiles occasionally flash red and yellow as if they were on fire. The magma flow tiles appear to function exactly like normal floor, as dwarves will pathfind and walk through them with no effects, and items will sit on them without falling.
The tile below the one being channeled (the semi-molten rock tile) becomes a ramp of the normal stone type for that area only if the tile was previously unrevealed. If the tile was previously revealed, it remains as semi-molten rock.
Placing and removing a floor on the magma flow tile turns it into a rough floor of the surrounding stone type, just as if the tile had been mined out normally. The tile below the magma flow is unaffected, so the floor can then be channeled out to expose the top of the ramp. A down ramp appears when viewing the upper level, but I have not been able to get the dwarves to actually use the ramp.