Though what you say about measuring through a feedline is true, is
there some reason you limited it to 1/2 wavelength? I regularly make
measurements through an arbitrary length of transmission line--though
one key is that the line not have too much loss. And of course, in
making antenna measurements, it's important to realize that the
feedline you use for measurement (just as the one you use to feed the
antenna) may couple to the antenna and become part of the antenna.
There may be some advantage in using the same line for both measurement
and using the antenna as an antenna.

For the original poster: yes, an antenna system consists of what you
thought was the antenna, plus its environment. People around the
antenna, or big vector network analyzers around the antenna, or
feedlines, or masts--any of that may affect the measured impedance. On
the other hand, if you measure the antenna in the environment in which
it will be used, assuming you make accurate measurements, you should be
fine. You can even experiment to see just HOW MUCH effect a person
near the antenna will have. Modern analyzers make this very easy and
quick. Ah, for the "good old days" of hand-balanced bridges, measuring
one frequency at a time! But what people actually use to make the
measurement depends on the equipment they have available. Do you have
specific equipment in mind?

Cheers,
Tom