@Dennis most useful for interactive testing would be to accept the program in octal, but "internally" do the hexdump translation and display the byte count from that;
here's a compiler from octal to 7's hexdump format that can be used for testing. (Note that 7 will infer the encoding from the source – it's treated as octal if it matches
/^[01234567 \n\t\r\f]*$/
– so I guess it wouldn't be impossible for TIO to do the same.)