Does this work with nesting? I guess, if reading the envelope gets you a pointer to the inner buffer, you can pass that to another read operation. If that can be done safely (with the library ensuring the appropriate checks before it casts/transmutes), that would be very powerful.

It should work fine. In C and C++, it's straightforward to YOLO it -- all you need is a pointer and a length. Rust can do more or less the same thing but with compiler-enforced safety. Many GC/ARC languages can efficiently handle slicing buffers, and it mostly comes down to library design. (Even Python can do this, although you generally pay for it in the rather large cost of every other operation...)