The only tool I can think of right now is moxie0's tool https://github.com/moxie0/knockknock back in 2011.
About the TCP port, if it's already open, you'll just have to wait for the next HMAC because of the very limited communication.
Finally, the choice of md5 was made because of the 16 Bytes output (knockles uses 10 of them), but don't hesitate to make a pull request!
Given that there's really no major performance/security benefit it's totally fine to use md5 imo, I shouldn't have even mentioned it in retrospect lol I just see "md5" and immediately my brain is like "hmmm".
Collision chance is still pretty low, my guess is that in practice unless you're doing something automated that performs hundreds of SSH's a day you won't run into it (30 seconds of "collision time" every 2-4 hours, based on my math).
Anyway, yeah, super into this.