No problem.
I've always captured my water but I don't see a technical problem using the cap with a garden hose. I've thought about a much smaller version of a gray water tote (like a 6 gal Aquatainer) and the garden hose adapter.
I only got those caps because that's all that was available at the time.
Look up under before spending much time researching holding tanks to add on. There's cables, floor braces, fresh water tank and the roof lift mechanism that are all blocking areas that you could install a gray holding tank. The hinges of that carrier I show above is actually anchored to the cover of the whiffletree box. That's all that was available.