Merging multisite sub-sites into one with nice redirects

After reading Jon's post today it got me to thinking about a project I've had on the back burner for a while.

I have a multisite setup where the main network site is just a placeholder and I have several sub-sites (completely different URLs). Mostly they're all just idle projects that never amounted to anything, however, a couple of posts have generated a bit of traffic over the years and I'd like to not lose that.

The bulk of what's in there can/could be deleted (and probably will) except for the few posts that get hits.

My aim is to totally remove one sub-site and transfer those posts to a different sub-site (the old sub-site is a sub-domain of another site and will eventually be totally removed). But most redirect plugins I have found deal with redirects WITHIN a site. What I'd like to do is generate a list of URLs from the old site and store/utilise those in particular to point to the new posts. Others would redirect to the associated category or tag, ie if the particular URL IS in my list of redirections, then redirect to the new post. If it's not, redirect to the category/tag or at worst, a 404.

I'm aware this would need to be a network plugin or something at the network level since (eventually) the whole sub-site will be removed.

So my questions would be:

Has anyone done this?

Is there a plugin I have overlooked?

I would prefer not to use a plugin but design something purpose built from the ground up and am looking for pointers.