This needs some clarification. What exactly is a related post?

For example :
I’ve posted about ieCapture a few weeks ago. Danvine has done since this post a lot of modifications so I’ll post the new features of ieCapture and test it. As this post speaks about the same topic or follow a previous post you want to link it to the first post.

This kind of feature is quite hard to explain because you can do this by with a trackback to the first post or having deep and very specific categories

Thought this thru: keywords, definitely. And in a separated table.