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Race numbers should work fine as keywords. I shoot a lot of aviation and generally keyword the aircraft registration among other items.
I'm not sure about the burning to disc, but I know if you export for email, the default is to keep gps and keyword info with the file You have to tick the check box to strip that metadata out of the file. I would assume it would be the same burning to a disc, but why don't you try burning a few files to a blank CD and then re-import the burned files into a new folder to see if the metadata was burned properly.