Is it possible to modify FASM to execute code prior to completion of assembly process? I'd like to code macro-like routines in x86 for use at assembly time.

That's an interesting idea, and I was thinking about it already a long time ago, but the main problem with that was the fasm's portability. Also this would be a perhaps very exploitable feature.

Maybe, we have even talked about it in the past?

What if this feature supports only a subset of the CPU instructions? Things like not supporting any kind of branching instructions nor memory operands nor system instructions. In such case fasm would need to support register initalization before entering the assembly block.

Although this way reduces the usability a lot, at least we could have FPU/SSE operations that are not available in the interpreter layer.