I finally had time to pick up my brake cables, got everything hooked up, cracked open the garage door and rode all the way to... the mailbox! (sad trombone). Turns out that even with the tiny extra clearance the 35s were showing while parked, it just wasn't enough when I put weight on the bike, the tire would rub on the brake arms. Pretty bummed as I was hoping to enjoy some plush rides before worrying about a 650b conversion.
To reiterate, the frame itself DOES have decent clearance for 700x38s and I'm sure the right kind of brake could potentially accommodate it, but it just doesn't work with the Tektro R539s. Refusing to throw more money at other brakes that may or may not work, I'm currently regulated to running 700x28s as those are the only extra tires I have laying around. It's kinda crazy just how much smaller 28s are compared to even 32s.
The good news? I just sold a bike, so I now have the funding to make the 650b conversion happen sooner rather than later. I just need to decide on a rim, which is proving to be a bit difficult. The ideal candidate would be something like the Pacenti Brevet with it's classic look and machined brake track, but there's no way I'm doing the "pre-order" without having at least a rough timeline. The Velocity Quill is kind of appealing as a modern alternative, but dang they're pricey in the polished version. I may just end up with the 650b version of the Dyad unless anyone has some solid recommendations?