End feeding seems more difficult than center feeding.

Fairly easy. I always preferred the "gamma loop" type
matching device. On a low freq antenna, that loop could get a
bit large. On 20m, a 18-22 inch single turn loop would match it no
problem.
maybe less than that, cuz using the "standard" 22 inch loop for 20m,
you will usually tap around the mid point of the coil. So the coil
could
be shrunk down, to use more of it. I always used lengths of coax for
caps
if needed. "open stub" 100-120 pf is a good value for 20m.
You can also use a regular coil to match, but my gut feeling is the
single turn coil is a bit more efficient. I've never actually compared
though...
BTW...I consider WWV's antennas to be 1/4 wave ground planes being they
have elevated sloping 1/4 wave radials.
MK