So I hacked up this quick and dirty fix for the solution.
Every time a tagboard entry is posted, I have a script on my local machine that takes a local copy of the layer, with a line marked "INSERTTAGTEXTHERE", strips out that line, replaces it with the tagboard text, then sends the layer to the LJ server to compile (logging in if necessary).
The script is written in PHP, and uses MySQL to store the tagboard entries.
See this post for source code (as HTML for easy viewing, or in .zip or .tar.gz for actual use) and instructions.