RMS was probably right not to sign it though. I also believe producing a t-shirt with RMS depicted as the Che is not quite right.
On the other (left) hand, I'd definitely buy an ironic t-shirt of ESR as Che labeled "Free Software Liberator" and ask ESR to sign it, just to piss him off and provoke him to threaten to shoot me, like he did to Bruce Perens.
The irony of Eric S Raymond threatening someone else for behaving "like that kind of disruptive asshole in public" is rich -- very rich.
ESR's Death Threat Email to Bruce Perens:
https://lists.debian.org/debian-user/1999/04/msg00623.html
Bruce Perens Dead:
https://geekz.co.uk/lovesraymond/cat/bruce-perens/page/10
Terrorismistic:
Otherwise it's mere faux-pas, at worst.
What's more, you don't necessarily know everything about a person. I would not know anything about RMS's political opinions if I had not read his website a bit, I would only know him for his stance on free software.