With a tubing vertical you can run the coax up the inside of the lower
half to feed the dipole and use relays to switch between base feed and
center feed for changing bands... GAP does something similar with
traps (after assembling a GAP for a local ham I am not a fan of GAP
antennas, their basic idea is sound however) You will need a choke
coil right at the base and probably should bury the feedline into the
soil as deeply as you can trench it - at least 3 or 4 feet - and have
another choke a quarter wave back from the feed point... Lots of
work, not much extra cost... Should work OK...
Other way to do it (and my preference) is think like a Ringo Ranger
and stay with base feed... Insert a half wave, parallel feed line,
phasing section at the mid point and get the benefit of 2 quarter wave
in phase radiators on 20 meters... Short this phasing line out for 40
meters.. Again, run the relay wiring inside the lower tube...

denny - k8do
In business as "Solutions are Us" - as long as I don't have to
actually do the work!