Sorry to be a bore (could it be worth than that?), but wouldn't fixing the program such as to accept relative paths be a better option? I didn't have the time to look into what Flex Ant task does exactly, but I wouldn't be surprised that instead of some sort of Path.combine() it does simple string concatenation / trims the strings.
Besides, I'm not certain, but why does it need to set flash player location? Best. Oleg