Well.....
I would pursue one of the following:
1) use a multisection power splitter with broader bandwidth, so that a slight shift in match frequency does not bother me,
2) Use a ground plane on the bottom of the dielectric, instead of relying on the metal surface as being your ground plane
3) use something other than microstrip, so that a ground plane is "not needed". Something like a center conductor and a co-planar ground sheet to the left and to the right of it.
4) use differential conductors, like coplanar strips, where the metal cylinder is not seen as a ground plane at all.
5) attach the dielectric to the metal cylinder with something like sticky foam tape, so that the spacing is constant during bending.
Rich