Hey Guys, A few things on several posts - I use the 1dc and smallhd 501with sidefinder as well (just added it to the mix and have been through 5x evf mounts to find the perfect one - it's not easy.
In terms of focusing while shooting, like any focus assist, the smallhd is hard to rely on too much when you're wide open, it's among the best I've tried, but it still shows some areas in focus that aren't. While you can't punch in while you're shooting on the 1dc, you CAN autofocus while you're shooting. You can use the af button on the back, and yes, it will temporarily wreck your shot for the few seconds it focuses and hunts, but it is MUCH faster than stopping rolling, punching in and then slating again. Even if you don't slate it's way faster. Here's a guide to doing this: http://bit.ly/2l392zk (it's the same as 5dm2 - many other eos cameras allow this as well, ie: 5dm3, 1dx.)
Also there's are a few tools which make it easier - I'm testing out the usefulness of this Bogen Sympla mvr911eccn wired remote that allows zoom and focus adjustments while shooting via the usb port - it's not perfect, it can be choppy, but it's tweak-able and is far less noticeable if you're wide and handheld. There are other more expensive tools which will also do this, maybe they're better?
All this said, I'd love for all of you to please also stop by contribute and help me out with my 1dc blog that's for all the intricacies of this camera (like the above) and how to maximize it's use because - that image quality, doe! And pease share your hits misses, gear loves and hates for accessories, share your work shot on the 1dc/1dx and help out everyone else when you can! i'm not monetizing it at all, this is a purely altruistic venture based on my constant frustration at there being very little info out there for the things I want to know: https://canon1dcuser.wordpress.com/