How much opportunity is their really in these small custom builders? It seems like they probably have designers they are happy with (judging by the product), and it doesn't take too many people on the business end to run a company with so low of production numbers? Therefore, I would think the only real chance at working there is the fiberglass laying and rigging depts, which you probably aren't interested in.
I think if you really want in the industry you have to start with someone like Brunswick who owns large volume companies such as Sea Ray, Baja, etc. Once you've proven your skill there then you could try and move to a custom race builder on the design or business side. I still would picture them as being more of a family run style business though.