Multiple Tag Systems or A Different Solution?

Is there a way of having two different sets of tags working on a blog at once? Perhaps hacking a plug-in tagging system to work alongside – but seperate to – WP’s own native tags, or some hack on the code that changes the naming convention of a “Tag” slightly, creating two distinct systems?

This is a very complicated requirement, but I think the solution must be really simple. Please persevere through my explanation.

I run a website at http://www.elephantwords.co.uk, which features daily writing on a weekly theme.

What I want to do is set up a sidebar thing that shows the other posts in a week when a post from that week is displayed.

I’ve looked at the Sets plugin, and also at the idea of setting up a sub-category for each week, but both solutions get very obtrusive or opaque – each of the six contributors posts their own work, so it needs to be straightforward.

I’ve also looked at going in and manually tagging each item with a numerical tag for each specific week, and have had some success with that.

The only drawback is, this renders the tagging system useless for ACTUAL tagging purposes, which some of the contributors use, and I’d rather not restrict that.

It seems like it should be something that’s just a matter of duplicating and editing code, but I wouldn’t know where to start…