i used to use Combat boots for overseas travel where i know
there will be trekking.
but since stopped when i realised it has kept triggering metal
detectors and gave up because there are some places that require
"no footwear"
as for duffel/field pack, i don't see a problem my only worry is
that some Customs officer might conveniently assume you are some
security risk and stop you for further checking.
my friend had quite a problem with US customs when they found
out he was SAF officer, but nothing much, just pulled one side and
ask further questions. no real delay.