So '"%...x",str' would be equivalent to '"%@...x",values(str)' for any value of '...' I take it? I.e. you would use 'sprintf("%02x", "Hej")' as a shorthand for 'sprintf("%@02x", values("Hej"))'?

That's what I assume, though I'd prefer

sprintf("%...x", Gmp.bignum(str, 256))

as equivalent in the first case (consider eg sprintf("%-016x", str)). The '-' flag should probably indicate reverse network byte order (analogous to eg %c and %F). Then there's of course the problem of what the format should do if str is wide?