If you can't modify the installer, then the only option I see is to parse the install script, although this seems sort of ridiculous. It sounds like you can view the code of the installer, so parsing the code shouldn't be too difficult. Just be sure to do some sort of checking so you don't eval malicious code.
The only problem being that this is future-proofing for when the install script *changes* - it may be just the data, or the way it's organised, in which case, all parsing bets may be off.
Ben.