it appears to use a printf function to make its output string

That is true, but its implementation in XS is pretty simple and can be adapted to suit your needs. Given the performance constraints you've described, I think you're going to have to venture in the direction of C/XS anyway.

Update: In fact, it turns out to be pretty easy!

use warnings;
use strict;
use B 'hash';
use Inline C => <<'_C_';
U32 myhash(SV* sv) {
    STRLEN len;
    U32 hash = 0;
    const char *s = SvPVbyte(sv, len);
    PERL_HASH(hash, s, len);
    return hash;
}
_C_

print hash("foo"), "\n";
printf "%#x\n", myhash("foo");

__END__

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