.htaccess is pretty powerful, but there are things it simply cannot do. You might want to think about routing requests to an active page for processing.
Here's something else that might work. This is something I had to do on CODmb.com
, a forum for Call of Duty
players. It's not an ideal fix, but it works. When we converted the board we didn't want to lose all the current links into the board. So what we did was save the old post/user IDs, detect that pattern, and reroute the request to a php file to build the right URL.
The SMF board on CODmb.com was converted from a bespoke message board with a very different database structure. When we converted the posts and topics we saved all the post and topic IDs from the original table and mapped them to the new post and topic IDs in the SMF messages table.
We saved all this stuff in a new table, so we have a table that has the before and after IDs to the posts.
The old board used 2 files named 'view_topic.php' and view_posts.php' to view things. We wrote our own versions of those two files and all they do is look up the old post/topic info, build a new URL for SMF, and send the user to it.
The downside is that those old URLs and the table will persist for years. Even they're aren't valid URLs they still work. Keeping the files and table there is no big deal, but it's clutter. Like I said, it's not a perfect solution, but it works.
Perhaps you could do something similar on your site?