I think I've cracked the texture issue, after trying the idea mentioned a page ago.
I'm balancing the meat mix by adding butter (in this case 5% total weight.) The butter gets worked into a compound butter with 0.7% salt and 0.2% black pepper, relative to the weight of the meat. Roll the butter into a small cylinder with plastic wrap and freeze.
When the meat is partially frozen, distribute the various cuts randomly on a tray. Cut the compound butter into small cubes and distribute evenly with the meat. Drop the butter cubes into the grinder along with the meat, striving for an even distribution.
Cook S.V. as you normally would. That's it. The result was perfect seasoning, no need for any last-minute seasoning of the patties, and no noticeable toughening from the salt (done without any kind of control; I can't promise there's no change in texture).
That's it. I believe the butter keeps the salt sequestered from the meat proteins long enough to prevent much curing to happen.
I did this with a blend of 1/3 chuck, 1/3 brisket, 1/3 oxtail. Next time I may try shin instead of oxtail (cheaper and easier) and add the marrow to the mix to compensate for the cut's leanness.