Re: Distinguish steel from terrain, if all else fails, could you not just generate a new bitmap on-the-spot based on testing each pixel for (non-solid, solid, steel)? Or if this causes too much of a performance hit, a possible alternative is to generate this map at the start of the level, then update it any time a destructive / constructive skill is used just as is done for the visual map?
(Of course, I don't know Lix's internal workings in regard to testing terrain solidity / steelness. This was a viable approach for NeoLemmix. If looking at NeoLemmix's source code here, you'll want to look at the LemRenderHelpers.pas file primarily, in particular TRenderBitmaps.CombineTo and TRenderBitmaps.DrawClearPhysicsTerrain.)