I tried to figure this one out. You could put a DP/DT toggle between the output of the 5 position switch and the input of the volume pot. The other leg would send the signal to the buffer.
However, when you remove the buffer you're sending your pickups to a 25K Log pot direct. This puts a strangle-hold on your tone. I can't speak to whether or not the 25k log pot can be substituted, but I suspect it would be another trade off. You'd get the direct to p/u line functioning, and lose the great functionality of the OBEL.
There are smarter folks than I here. That's what my futzing about has shown.
"Hi Ho...the Carrion Crow...FoldeRoldeRiddle"