I have done the Accucraft 3 truck shay. It was actually my first DCC install. The main problem for the open cab 2 truck could be the limited speaker size. Jonathan at rctrains.com has some nice small ones that would fit and still give good sound.
The 3 truck shay was straight forward. I turned the loco on it's left side and then unscrewed the bottom plate to access the wires for the power pickup and motor. I removed the existing card and pitched it (but before this make sure you know which wires are which). Hooked up the left and right power pickups to the motor decoder. Then from the motor decoder to the + and - to the motor wires. Hooked up the front, rear and cab lights to the decoder (the wires were all connected to the existing board so it was easy to get at them before I pitched it).
I installed the sound decoder and speaker in the trailing truck, but you won't have that option, but if you can get the front of the smokebox off you should have room for a Phoenix P5 and small speaker. Jonathan is the expert and Accucraft referred me to him as he seems to know more about the Shays than they do.
It is quite possible to install Straight DCC in the open cab shay. i have done full onboard batteries, p5 and Airwire in a bunch of them . I have a custom speaker which is a snug perfect fit in the bottom brass turning of the t Boiler. The only complication is that you need to move the lighting regulator to fit the speaker. The upper portion of this boiler and the front of the smokebox area will have enough room for a 2-3 amp DCC decoder. There is no room in the bunker in back since the motor is vertical and uses up the space there. Once the boiler is sealed up again the sound is outstanding.