Sorry to double-reply, but this is actually a super important point in favor of Fil-C.

If you adopted Fil-C for apt, then you could adopt it optionally - only on ports that had a Fil-C compiler. Your apt code would work just as well in Fil-C as in Yolo-C. It's not hard to do that. I think about half the software I "ported" to Fil-C worked out of the box, and in those cases where I had to make changes, they're the sort of changes you could upstream and maintain the software for both Fil-C and Yolo-C.

So, with Fil-C, there would be no need to ruffle feathers by telling port maintainers to support a new toolchain!