Can the now expired iMessage app still work with Lion?

Solution 1:

No one has come forth with a hack or modification to get it working. The longer it takes, the less likely someone will want to release a patch since the audience for this shrinks as people upgrade and move on to Mountain Lion.

Solution 2:

As on my own experience, the version you're talking about was very buggy and Apple did change a lot regarding iMessage on the new OS. It most likely will use other protocols or URLS to communicate with, and as it have been replaced by a full (working) version it's no worth to anyone to attempt and hack it.

So in short: No, you can't do this. But you might be able to put Mountain Lion on your older Macbook.