Yes, I do you think you could say that you were staying at a hotel, and ask them to pick you up the dock. The problem is that the hotels are mostly on Paradise Island, where the blue Lagoon ferry does pick people up. So you would need to get from the crew ship dock over to the paradise island dock.
In my experience in going to Blue Lagoon 4 times, we have gotten on the ferry inside the gated port area. So the only people able to get on the ferry at the cruise ship wharf are cruise ship passengers.
There have always been multiple times to catch the return ferry, so it's up to you to make sure you time it so that you're back by the time the ship leaves. The ferries to Blue Lagoon are large, I would guess they hold a 150 people, so I don't think you would need to be concerned about them reserving a ferry for cruise ship guests only. Each time when taking the return ferry, it is always stopped at the Paradise Island dock. When returning to the cruise ship dock you are required to leave the gated area and come through the secure area to have your identification checked.