How to find a frame given a geometry
Getting a framebuilder to put horizontal dropouts on the Boardwalk looks quite possible from a quick glance at the picture. On the other hand, it looks as though it has fairly conventional geometry so you may find that a current bike with hub gears off the shelf works for you. It's likely to cost more than $300 to get new dropouts fitted since you'll also be stripping and repainting the frame. Since it's alumnium and most framebuilders work with steel you might struggle to find someone who can do the work.

An alternative is to fit a hub and use a derailleur or chain tensioner to take up the small amount of extra chain you'll need. chain tensioners are common on singlespeed bikes for the same reason as you need one (and singlespeed conversions are very common). This will be the cheapest way to fit hub gears to your current bike.