I had a similar issue, in that I wanted my entire above-ground fort to be engraved stone, and also not obsidian. You have two major options, both involve hacking. First save your fort before you mod raws.
1) Remove the [LAVA] tag, and place it on any other stone, soil, or mineral. Now, when you combine water and magma, this material will appear as an engravable stone.
2) Remove the name OBSIDIAN from the entry for obsidian, name it whatever you want. Find your desired material, and name it OBSIDIAN instead. Run DFHack, and use DFLiquids to pain obsidian. It picks the entry in the raws for the name OBSIDIAN, so whatever currently bears this name will be placed. Keep in mind that a wall crated this way has no floor above it, so you need to place "obsidian" on the floor above it also to make it a real "wall".
Alternatively, move the [LAVA] tag where you want, and then use DFLiquids to place a square of magma with f- then next to it (like in the hallway) place a square of water with f+ This will make the magma remain still, as it's in a non-flowing state, and the water will flow out from its position, touch the magma, and cast obsidian (or what have you). You can then smooth the floor to get rid of the resulting mud, or place a cabinet/table/chair/etc on the mud to remove it.
Coincidentally, I swapped the [LAVA] token and the OBSIDIAN name with RAW_ADAMANTIUM and ran the game. Using DFHack, I could then paint adamantine walls that could be smoothed and engraved. However, when I breached the magma sea, I got the curious message "Obsidian! Praise the miners!" and there were spires of obsidian, even though obsidian itself lacked the deep_special tag. This was very strange, but not an issue because I had acres of adamantine already.
You know, there's a way to accomplish that 'legitimately', i.e. without DFHack or changing any raws. You just need to find a spire, one of those square/circular mountains contained in a single tile, surrounded by flatland (could take a few embarks to locate one, but not too difficult with the help of the cliff indicator/relative elevation tabs). Center your embark on it and carve your fort inside the spire, then channel away all the excess dirt/stone (if any) covering your fort, and voila - you have an aboveground engraved natural stone fort towering over a flat plain, no sign of the original spire. Of course if you have other conditions - say you want a spire that
also has metal and flux and whatever else, finding that perfect embark could be tricky. I'd say it's worthwhile though just as a matter of personal preference, to have a 'natural' aboveground engraved fort instead of a hacked one.