The travel call would definitely stand. It's a travel. If a defender
chooses to make that call, the rules support it and that's the end of
it. No implicit agreements (regarding physicality, ground touch
alternatives, or otherwise) will trump the rules or undermine a call
supported by the rules..
Practically speaking, I agree with Jon's comments above, in that lots
of players accept the offering of the disc as a ground-touch
substitute and it accomplishes the purposes of the ground touch. If
the thrower and the seven potential travel-callers (defenders) are all
on the same page, then that seems fine. But unlike agreements between
opponents on physicality, which require only two players to be on the
same page, eight players need to be on the same page for the ground-
touch alternative.
I always just touch the disc to the ground, though. It avoids
confusion and I see no real good reason not to. It also gives players
downfield a clear indication that the disc is in play. As a marker, I
do not always set up close and in comfortable disc-touching/offering
range, so I prefer the thrower just touching the disc to the ground,
rather than walking up closer to me (to the incorrect spot) to offer
the disc.
-Colin