I agree with Thor in regards to the cost effectiveness of doing what you plan with your Dahon Boardwalk, if approaching it from the angle of simply wanting the bike, and purchasing parts through retail.

You can buy a Mu P24 with a dual drive, V brakes, and much better components *absolutely everywhere* on the bike for $650.

There are also lots of 2008 Speed TR's available for about $720 and up that have the dual drive, big apples, v-brakes, much better components on the bike globally, and full equipment with rack - fenders - lights - pump

Either bike is getting you much better value for your money if building the bike up through retail channels, when you factor in the end cost of modding a boardwalk to the degree you are considering.

For your Boardwalk, you're going to need to get the dual drive, spokes/nipples, new rim if you want a decent one, the DD shifter, possibly a clickbox, a new 3/32" chain, cassette, crankset on the Boardwalk 1/8", not 3/32"? probably. If so, you're going to need a new crankset to work with your 3/32" multi-speed chain as well, and the derailleur itself.

braking components including the brakes, cable and levers themselves, and the mounting hardware

Not to mention the custom adaptation you're going to need. Respacing, fitting potentially custom mounting hardware, building your wheel or paying to have it done. I'm also wondering how you will mount the derailleur needed to operate your dual drive hub, since the Boardwalk does not have any facilities for a derailleur. Nearly all of the clamp on derailleurs are utter junk, leaving you with the prospect of either suffering low quality equipment or coming up with another mounting solution.

I'm not going to delve too much into breaking down the costs, but as a rough example, your bike @ $200, Big Apples @ $70, Dual Drive hub @ $240, you're still missing a lot of parts and the price of the bike takes a huge jump. By the time you've added the rest of it, the bike is well over the price of either of the other example bikes, and still an inferior product.

Even if you're buying this stuff online at the lowest prices you can find, I personally doubt you'll get the price of the bike reasonably under the $650-720 of the other 2 Dahons, and it will still be an inferior bike in regards to the rest of the components. The only way to come out of it with a relatively low budget that I can think of would be to find used / bid goods on ebay, or to retrofit another standard IGH into a dual drive, as some have done.

Now, when I see that you mention a "custom" dual drive hub though, I am wondering if you intend to be approaching this bike from a project perspective, and you really want to enjoy building the bike as you go. In which case, it could be a fun challenge. If you can convert your own dual drive, machine your own brake adapters and find inexensive ebay / lightly used parts or visit a local bike co-op, you could potentially scrap together a nifty little thing quite well. Regarding fitting a derailleur, there were some clamp on "plates" I have worked with occasionally that had a mounting for a hanger mount derailleur on them instead of being attached to their own unit. That would let you use a decent derailleur with this bike, so long as the clamp didn't interfere with the IGH.