One other comment:
If you are on their property and you haven't agreed to a liability waiver, it's a huge issue for them. What if you are coming down and you hurt yourself on a trail and then you sue them for not having a safe trail? The liability waiver on every ticket is in place to protect them from that risk.
At Cypress Mountain, if you want to access Black Mountain, Bowen Lookout, or other trails in the provincial park, you have to cross through a very short section of active ski resort (they don't own the land but they have an operating certificate to run a resort in that area of the park.) As a result, you must go to the service desk and get a free pass that basically just has a waiver on it saying that you won't sue if you get hurt during the time you cross the short section of active ski area.
If Whistler was going to allow uphill bike travel within the area of their resort, they would have a similar program set up (kind of like the program they have for one-ride-up skiiers in the winter.) They don't have that program - you can't offer to sign a waiver.
Let us know how your conversation with them goes but I don't think they will be ok with it.