Right, I’ve a thought, but you might not like it…. Can the inner habito section be dismantled? If so, get that out the way so you can get a good access to the block work, then mechanically fix a length of timber to it. Or perhaps a 15mm*25mm angle but flipped inwards so you have the 15mm facing inwards, bond or mechanically fix to the frame. The reveal board when plastered will cover the metal.
me, I’d look to my first option.
I think @Iceverge may have had solutions for tricky reveals in the past? And @Nickfromwales may have seen a situation like this out in the wild. Sorry for tagging you chaps, this one needs more than my willing head!