There is u32::from_str_radix(input, 8), which does not specifically handle the "0o" prefix but that should be workable.
Edit: This is oddly an on-topic question for this forum, since you are proposing an addition to the standard library, but if you were only wondering how to parse octal in rust then you're probably looking for https://users.rust-lang.org/
Adding an alternative method that looks for the prefix to decide the radix sounds useful, though. (But I don't know if libs-api would want it in core, vs leaving it for a crate to provide.)
The strtol family of functions in C autodetect the radix if the radix argument is zero, but of course such magic values aren’t very Rusty. A const BY_PREFIX: u32 = /* implementation detail */ might work, but yeah, a separate method would probably be the way to go.