Or I could rely on SBCL being used a whole lot and indeed having immutable such default syntax tables with understandable messages to blame whoever tries to mutate those tables without rebinding them
this approach seems to be the most straightforward to me.
especially since we have that great cl-test-grid running on multiple lisps, one of which is sbcl which will catch the wrongdoers.
i'd also argue that it's also a potential time waster if asdf rebinds a copy and someone foolhardy tries to modify *readtable*, and will experience zero sideffects contrary to his expectations.
he will waste his own time only, so it's already much better, but still not ideal.