I have access to a syntax highlighting program for an internal-only language. Is there any way in which I could get vim to call this program to perform syntax highlighting? I figure that it's worth seeing if this is possible before I write my own syntax file, since it's quite complex (not to mention prone to change). The program in question could be quite easily customized to output in a new intermediate format (it's only currently outputting HTML), if that would make things easier.
I would suggest to write a quick throwaway script in the language of your choice to convert the output of the program in question to the Vim highlighting syntax. You mentioned the program is prone to change, but its output format can be easily customized: using your own conversion script (which parses the programs output format) you could easily stay up-to-date with the latest changes (just run your script again). And as others have mentioned: creating a new syntax file is really easy in Vim, so it's up to the complexity of the programs output how hard this actually is.