So, flushed with the success of setting up the RS8V I attacked the iffy Mockingbird. I slackened off the truss rod to give some relief then lowered the nut height a little. This time I attacked the nut shelf as it was more substantial than the RS8V and made of wood rather than weird plasticky stuff.
This has really helped and made it much more playable. I'll have to take a bit more off the nut height but I've decided the monkey metal tremolo really has to go. I'll order up a new one and do both at the same time.
With it approaching playable I went for the electrics. I ditched the neck P90 and Epiphone bridge then fitted a matched pair of Kent Armstrong 'rockers' I bought ages ago cheaply. I had written off the idea of using them in this as the bridge wasn't bought as F-spaced but when I actually measured it's actually pretty close to it. In my usual fashion I stuck in some push/pull pots and wired it for coil tap. Given some 4-wire pickups it's almost rude not to.
With it all back together it's a lot closer to what I wanted from the Mockingbird when I started modding it, but there's still a way to go.