After re-reading this thread, and thinking about the scooping technique, I have modified the housings I do. With the new 3/4" insert bit I'm using, it is not difficult. The router is being used with a 30mm template guide, which leaves a good 1/4" border around the housing template I am using. I can easily cut the housing 1/8" deeper except for the last outside 1/4" which would be as acurate as possible. This would be similar to your scooping done with your hand tools and hopefully have a similar effect. I think.
As far as gaps from shrinking, I was lucky enough before doing any traditional timber joinery, to get the best advice from a local old german timber framer. He used to cut the timber hybrid components in the Mcmansions ( I hate that term, because there are some really well built homes) I was framing. He told me to make sure what I used was really dry. It seams though that that can be easier said than done, especially given a person's location and local species. I.suppose