I say go for it. Competition is a good thing, it breeds innovation.
As a small business owner myself, I understand startup of a new venture can be expensive. While it does take some money to make money, using a resource like MUD to help defray startup cost is a wise decision.
Even if one quality top on average a week can be made, for the same amount others charge, that's 52 tops a year in the hands of folks that need them. If demand ramps up, then he can add additional resources.
Another vote for the zip out windows and all stainless hardware.
Another idea for you:
1. How about a zip on 'awning' to extend the top out to one side or the other or both? Would zip on at the top and come with poles to support the other end. Would be great for instant shade to eat lunch, work on something out of the sun or to sleep under. ( I'll take 10% of all the profits for this idea!

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2. Storage bag for the whole top, and a smaller bag just for the zipped out windows or awning, would be cool options.
BTW, does your brother-in-law ever work with Aqualon fabric? My pop-up camper uses that material for the tenting. It's very flexible, and it's tough stuff. Totally water proof, available in many colors, easy to patch blah blah blah...
I'll be in the market for a top in about a year so I'll monitor this!