>> Would the canonical tag (and its ability to be cross domain) perhaps be an option?
Google can be fickle. Normally I would say using that might be an option, but Google is so random right now it's hard to say.
Either way, I would probably plead with the manufacturer and propose some kind of deal with them to continue using the domain name. If your client is promoting the product, why the heck not? If that doesn't work, do a 301 redirect and forget about the canonical tag. Second suggestion would be to use the canonical tag for a month or two and then do a 301, but keep the original domain name for as long as possible to maintain those 301s.