For example, I have some Eastern Hemlock rootstock (Pinaceae), and want to graft a scion from Dawn Redwood (Cupressaceae) onto it. Is this possible? Is anything like this possible?
No. Exceptions are always possible, but it would be a noteworthy.
I just find this notes in: https://courses.cit.cornell.edu/hort494/mg/specific.grafting/compatibility.html
Although there are a few reports of short-term graft union formation between distantly related (interfamilial) herbaceous species, there are no confirmed reports of interfamilial grafts between woody perennials.
One extremely curious report of natural interfamilial graft compatibility which must be considered unconfirmed, and even unlikely, was the observation of La Rue (1934) [...]