I don’t you’ll have success syncing Tinderbox with a DEVONthink Smart Group – smart groups are saved searches and have nothing real inside of them – no child documents – at all. Ever. They only display “contents” when you click on them in DEVONthink.
At this point, as @mwra pointed out, this is experimental. The techniques used for syncing a group are different from the techniques used when you drag documents from DEVONthink to Tinderbox, and there is not yet total conformance on how attributes and contents get populated.
I’m sure with input from this forum’s DEVONthink users (as I am) things will evolve.
Absolutely. The “Watched” feature in 7.3+ is intended to help “get info into Tinderbox” – it is not intended to make Tinderbox into a file hub of any kind. That task is best left to DEVONthink or KeepIT, etc., that are purpose-built to act as file managers.