I have two thoughts.
Second, I don't see a market for it. I agree that generally, things that make the game easier could find a market, and maybe belly putters are one of those things. But if you're in the category of people looking for help through illegal clubs, you're probably a terrible golfer. Box grooves and bigger driver heads aren't going to make any difference. Maybe belly putters. But if that's all we're talking about then nobody should care. Its not like you're going to get cheated by a guy who anchors without you knowing.
I think the size of the market will be relative to the perceived / actual advantages they provide. A driver is not just limited in terms of size but the trampoline effect is also restricted, so woods that conform in size but use special alloys to allow for an additional 20+ yards per club might attract attention. Wedge / ball combo's that allow slower swing speeds to spin the ball more easily might also be seen as useful. You already mentioned anchored putting, but there are also other non-conforming putters on the market, one that looks more like a croquet club than a putter, and one that uses a laser "as a training aide".
As stated earlier, aluminum bats that are not permitted for league use still are offered by many of the major manufacturers so there must be some market.