Wheeeeee, that was much simpler than I thought
As i'm quite new to Actions Chain PI, I hadn't noticed the "Set to current time" option.
Also, I think that, as I was used to bring up the Edit Metadata dialogue to set #fav value to 'yes', and only recently have started using just the shortcut key assigned to that Action, I didn't even consider including another action to the chain.
I thought this would only be feasible by adding some mysterious template language code to a custom column built from other columns.
