Tricky, as getting hold of magma is also often reliant upon digging. (Magma-safe pumps need suitable raw materials for workshop(s) and the components and perhaps power-source components, but as you seem determined to carve fortifications, it sounds like you're even more minimally equipped.)
I'm going to hope, then, that you have enough raw materials to build a floor over the ice, and enough guiding walls to keep the magma on that whilst still (somehow) getting side-access to the meltwater. I can't right now picture if the heat transmission will spread far enough (heat down, through floor, then melts sideways sideways at least two tiles to get under the retaining wall and then at least one 'open' tile.?). At least with underfloor heating, you know you have access to directly above the meltwater.
And if you're block(/rock/log/etc)-light, we're talking an absolute minimum of two units of materials (one floor, one blocking wall, and assume the magma doesn't spill around that wall to stop access to the hypothetical meltwater) or a very lucky spread of free-flowing magma. Possibly, if the terrain is conducive, you could let the magma drip through a natural ice rill, or against the side of an ice-slope, obsidianising its own wall and (hopefully) melting the ice the other side without overflowing it... But it'd need some specific conjunctions of ground features to do what I'm thinking of here. Is there a screenshot or diagram of the locale we can look at?
(And was this a minimalist embark, or the result of an accident in which you lost the usual standard equipment? Either way, good luck!)
((Might be ninjaed, whilst I was thinking hard about your problem...))