Adapting the new masking tools to my previous workflow (Same brush across many images)

As a broader comment really, whatever were found to be the best workflow opportunities for how the software used to work, will always be denied us once the software changes how it works. 

So we need to find the workflow opportunities that deliver the most efficiency how the software works now.

It's just a matter of identifying what those new opportunities are. They are bound to involve different workflow steps, and/or in a different working order, possibly differently conceived. Such is change. 

As an example, say this brushing that would have been done on each image in turn, was so as to apply [some adjustment] to the background areas of the portrait. Perhaps one can now as a bulk task tell LrC go and find the background areas within each portrait all by itself, and apply [some adjustment] to all of those in a smart manner. Perhaps a number of standard masks (pre-loaded with their respecitive adjustments and named for ease of reference), may be bulk-applied together so that then only tweaks would be needed within each image, which would then make use of standardised named masks which were all already in place.

Yes, agreed, the K key helps a lot. I would just comment that a particular working method tends to arise in use, naturally, out of how the tools work now. So how the OP is seeking to add first one kind of adjustment onto all the images, and then another kind of adjustment onto all the images, is of that sort - efficient under the tools as they were. But not the only efficient way to work. When the tools change, new ways to work will naturally arise from those and may even be more efficient. For example, syncing a whole set of pre-made adjustment masks, each named "skin" or "hair" or whatever and pre-loaded with the preset adjustment, across the images. Then within each image, you can activate these pre-made masks in turn, by name, and brush onto the image where appropriate: but you can do this freely and not in any 'forced' working sequence. 

But assuming the OP's preferred way of working - the sequence in which you choose settings is relevant.

So: say you press K, choose "skin" adjustments to make those the current default, then paint onto the image. Go to the next image. Press K, you can immediately paint the same "skin" adjustments here: they are now the current default. 

But: press K, start painting, this will begin with whatever adjustments were the prior default. Change to "skin" adjustments, those are now seen instead (but just for for this particular mask). Go to the next image, you are back to those older adjustments -  since they remain the default.

It is this persistence which makes it efficient to run through all the images in that particular systematic way. But by the same token it's a bit of a working straitjacket IMO.