Two great issues here
1. supporting a carbon push rod -- I don't think the pushrod will bend, but there's a stress point wherever the assembly goes from carbon to the pushrod ends. I found (the hard way!) that 2-56 hardware ain't strong enough to deal w/ the stresses, whether caused by harmonic vibes or whatever... flight #5 on a new Focus ended ...poorly... Same pushrod w/ 4-40 ends (same plane too after some major repairs) now has 200 or so flights on it. I do use a bearing at the tail of the airplane to support the rod aft of where it Y's out to the elevator halves.

2. The other issue -- stress induced by harmonic vibration - an excellent one. Any foam or balsa that constrains the pushrod's lateral movement will work I think - I've used both. One thing though - I DON'T think it's a good idea to put the vibration dampener at or near the 1/2 or 1/4 point - if the rod vibrates, chances are the 1/2 and 1/4 length points will be stationary nodes, and there'll be no movement there... just on either side of it.
This is one place where it's probably better NOT to measure precisely - I just put the damper somewhere near an end, NOT at the middle - and so far it's been a good solution.

-Rick