[rharbison]

While not a solution, here's a trick to make it easier.

1) Go into the options and set it so members can choose to display 100 threads per page.

2) Have your mods go into the user CP and set that as their default view, at least while they're splitting the threads, they can change it back later.

3) Click on the last page, and you'll see a list of threads. Note that the pages are now in nice even increments of 100.

Here's the trick I just discovered. At the very right side of the page, directly under where it says "Page 1 of X", there's a small down arrow. Click on that, and it says "From this page". One of the options is "Select All".

By using "select all" on each page, and having the page in increments of 100, it's pretty quick to split the thread.