and none of the above have solved preserving types across machine boundaries (or at least not in a stable/non-experimental let's-bet-the-bank-on-this release).

What about Static Pointers[0]? Do you feel the solve the problem or are likely to if you characterize them as non-stable and/or experimental?

0: https://ocharles.org.uk/blog/guest-posts/2014-12-23-static-p...