I would assume one could clear out most of it with a saw.
The router bit taking off a few mm or a few 1/16" would not be too bad on the bottom, but the sides seem like a lot of plunges would be required to get there... and a long lever arm, means you to not want to have a lot of force on the bit.
http://www.rockler.com/rockler-rout...nRaIs4f35lm5o6-xxFv29l-z0W6tDiSVcnRoCCVTw_wcB
I am not expert no this stuff. So I would use a saw and a slick.
You could probably have a home-made mitre-type template to aid in a Japanese pull-saw being held straight for the sides.
Maybe some plunge-saw to start the sides on or near a line, and then bang away with a chisel to remove the waste.
An auger bit and a drilling station could also potentially help to make some holes from the sides so you have a reference and a relieved corner to 'bang it out' to.