Using Multiple Merge Tools

I am trying to use Bazaar for development in C and LabView on Windows. The Labview files are in a binary format, but Labview provides diff and merge tools for working with these files. I was able to get the diffs to work using the --using flag, but I cannot figure out how to do the analogous thing for merges. I found the plugin extmerge, but it seems to only allow switching out the built-in merge tool with an external one, not switching between merge tools. This is particularly a problem since some projects will include both Labview files and C code (compiled to dlls, which are then loaded by Labview), i.e. I cannot just specify Labview merge tool in one branch and not in others. Is there some way to specify which merge tool to use, similar to merge-patterns in mercurial? Thanks!!