Icy Strait Zipline on your own?

[TwoCalBears]

We'd like to do the zipline when we're in Ice Strait on the Royal Princess in July, but the Princess-sponsored zipline excursion is scheduled at the same time as a Glacier Bay flight-seeing excursion we'd like to take. Since both excursions are filling up and we had to choose, we've signed up for the flight-seeing excursion, but were wondering if anyone knows if it's possible to do the zipline without signing up through the ship's excursion program (if so, we could do the zipline after we get back from flightseeing). I ask because if you access the Ziprider website through Icystrait.com, it looks as though the only way to sign up for the Ziprider experience is through your cruise ship's excursion department.