I don't think GPLv3 anti-DRM clauses would kick in here. They would if 1) someone was distributing a combination of a Secure Boot-only machine with your software installed on it, and 2) the loader (which is the only thing really validated by Secure Boot) would actually try to continue the chain of trust, and validate all the other bits, such that the user cannot run a modified version of your software.
I'm pretty sure that neither of those is the case, however. Once the system boots using the signed loader, it's really just Linux, and you're free to replace the kernel and any bit of userspace as usual.
Furthermore, I seriously doubt that anyone is selling machines with Linux preinstalled that have Secure Boot which cannot be turned off - simply because that would become known pretty fast, and even aside from licensing issues, would elicit a very hostile reaction from the community (and hence many potential buyers).
I'm pretty sure that neither of those is the case, however. Once the system boots using the signed loader, it's really just Linux, and you're free to replace the kernel and any bit of userspace as usual.
Furthermore, I seriously doubt that anyone is selling machines with Linux preinstalled that have Secure Boot which cannot be turned off - simply because that would become known pretty fast, and even aside from licensing issues, would elicit a very hostile reaction from the community (and hence many potential buyers).