I was looking for the same thing today, and Rod Jamieson was kind enough to send me a UDL file which he created many years ago. I've attached it here, although with some minor modifications. One is that I am used to starting comments with '::' (without the quotes), which is quicker to write than REM and is also less messy in the layout. Therefore I added this token to the comment section. Anyway, hope this works for you too.
Most of the syntax coloring work now correct. I just added a "%" (in prefix mode) to keyword group 5, to allow to colorize variables. One thing on the other hand is not working. If a internal function or a variable is prefixed by a character or number, it is not recognised as a function or variable any more. See attached example. I also attached my current definitions file (as .txt; .xml is not allowed).