Parsing from a std::span may be the right idea.

A solution to the "bad I/O problem" may be to standardize memory
mapping. In that event, the standard library would just abstract from
the OS and give you a byte array that represents a file. Whether you
use that byte array to parse JSON or do any other workload is up to
you then.

That would be a feature that belongs in a standard library, obviously.
It's something that would have to be optional on freestanding and
which would be fairly complicated to standardize, but it would have
tremendous value.