If Gentoo doesn't work on fridges, what's up with this:
> The Gentoo/s390 Project works to keep Gentoo the most up to date and fastest s390 distribution available.
That's a declaration of support, and if that's not what they mean, they could list some limitations on their wiki. [1]
A lot of system distributions declare some platforms as supported and others as best effort and still others as probably not working, but you're welcome to try. That's reasonable, of course, but it's nice if you're upfront about it.