I built an application for interacting with Apple iMessages on classic Macintosh systems via a modern Mac. I've tested the software on System 2.0 through System 6.0.8, but it should work on Macintoshes running up to MacOS 9.2.2.

In addition to the github repository, I wrote an introduction blog post about it here: https://henlin.net/2022/02/26/Introducing-Messages-for-Macin.... I've also outlined the development workflow on to other posts at https://henlin.net/2021/12/03/a-simplified-guide-to-developm... and https://henlin.net/2022/02/26/Messages-for-Macintosh-Develop....

Let me know if you have any questions or decide to try it out on your Macintosh, I'd love to hear about it.