Ollie, I think phenolic is easier on the blades than glass, but...phenolic is still very hard...and if it were me, I would save those nicely sharpened jointer blades and avoid it.
Another thing you can do is put the phenolic in the riser on an arc...then when you cut your limb pads and true them up on the jointer it will not interfere with the phenolic.
Also, I think a strip of wood will strenghthen the riser close to that of any other laminated material (glass or phenolic). (unless you are making an extremely heavy bow..or one with a very thin grip..I think it will be just fine)