Is the first version of the Message-Digest algorithm by Ronald Rivest publically available?
Since I have not received any reply from Mr. Rivest's office after bugging them with a total of four emails in four weeks, I have no other option than to give up on hoping I ever receive a reply from his office.

After spending 6 weeks hunting down information all over the internet (and not receiving any reply to my emails), I am currently suspecting that MD1 has indeed never been published. I base this on the fact that another algorithm — MD3 — never made it beyond the experimental status either… due to problems with its algorithm. It could well be MD1 faced similar issues and also never made it beyond the lab. Another reason for suspecting this is that all RFCs were published in April 1992. If there had been any "beyond lab" MD1, I'm pretty sure it would have been published together with the other MD papers. My best guess is that MD2 was born out of failed MD1 experiments. 

Yet, my suspicions might also be completely wrong. As said: I was not able to find any papers or hard facts in relation to MD1. Should you ever find reliable information about MD1, please post it as an answer. I will gladly unaccept my own answer to accept any answer that's able to present verifiable information and/or a publication/paper/book/whatever related to MD1.