The main problem was that my previous Xtracycle frame used a 27.2mm seatpost, which was small enough that a standard 1-1/8" stem could be attached to mount the stoker bars for my passengers. Like this:
The EdgeRunner frame specs said to use a 30.9mm seatpost, which I did, though I didn't think about how I would mount the stoker bars to that larger diameter. By looking at their site, I can see that Xtracycle has avoided this issue with their complete EdgeRunner bikes by using the smaller diameter seatpost and a shim. They obviously thought further ahead than me.
I rummaged through my spare parts and found a short stem that is also for 1-1/8" headsets, but clamps on a handlebar that is 31.8mm. So I flipped it around, mounted the handlebar end of the stem to the seatpost, and shimmed the stoker handlebar with bits of aluminum cans to get a snug fit. Like this:
My other issue was with the KickBack centerstand. The stand has a hard plastic plate that mates nicely with the round tubing of the FreeRadical when not in use, but the EdgeRunner has an oval shaped bottom tube which doesn't mate nicely with that same plastic plate. I bought one of the first KickBacks available and don't recall it coming with extra bits, but now I can see that Xtracycle provides the stand with plates for both tube shapes. I now have a oval shaped plate on order, but in the meantime I just wrapped the stand with an old inner tube to protect the EdgeRunner frame. Like this:
Lastly, the frame has a handy piece welded on specifically for mounting a fender on the small 20" rear wheel. The braze-ons use a slightly larger bolt than I have on-hand, so I just used cable ties and coroplast. Make sure it's not too wide on the drive side as the chain will rub when using the largest inner cogs. Like this:
With all of my nagging accessory issues out of the way, I get to just enjoy riding the bike.
Are you building up an EdgeRunner? Share a note or a link in the comments.