but I'm also pretty sure that they *don't* support everything that works with free software because of the rigidness of packages.
it could be true, but anyway it is less important than the fact of inserting a device and mark it as working with free software when some non free drivers has been used (even if it is not your case).
Restricting OS selection as h-node presently does really just forces people to provide misinformation for that field.
Yes, it could be true, one could say he has tested the device with Trisquel even if he has used Ubuntu or Suse, but I think that saying that those distro are not allowed is, in some way, a strong statement. Indeed you have done exactly so (tested with Gentoo and marked as Trisquel) but then you have noticed this fact in the issue page. And this issue is important to better specify and reflect upon the h-node policy.
For example people that usually use Ubuntu could feel harmed by the fact that that distribution (that they use every day) is not allowed here, and I believe that most of the them simply decide to ignore h-node.org because they can't submit hardware tested with their distro. But at the same time they have to consider this question: "why our distro is not allowed?" And the answer is: "because it is full of non-free software". And I think that, in some way, it is a very good way to highlights those distributions that have only free software and to show how things could be done in a different and better way. So even if someone could provide misinformation for that field, I think that that field could be useful to delimit, strong and increase a community of people that want use and share only free software.
About Gentoo: it is a very strange distro because it is not really a distro and someone (if competent) can build his operating system fully free. But really, it would be very difficult to understand when such a distro should be allowed (or not).