Disclaimer: I’m one of the founders of Ferrous Systems, the company behind Ferrocene.

One of the goals when certifying Ferrocene was that it serves as a drop in replacement for rustc. So while we’re happy if you start out building your product with Ferrocene (and have made our pricing model compatible with that) - going the route of “rustc first, then slot in Ferrocene” is entirely supported. There are also sometimes good reasons to pick that approach - Ferrocene is much more limited in terms of target support and while we may have a timeline to deliver the target you need in the qualification level you need, we might not ship it yet (though we usually can enable Support relatively quickly)

That said, I’m quite confident that using Ferrocene gives you a faster route to certification than trying on your own. I’d not be surprised if we hear from them.