You definitely don't have to be a guest of a participating resort to use a Minnie Van.
Download the Lyft app before your trip. When you visit BC/YC, Boardwalk, or WL, visit bell services to ask for activation. Keep the Lyft app closed. Bell services will ask for your mobile phone number and will send you a text message. When you then open the Lyft app, you'll see a Minnie Van (with cute graphic) as an option (as well as regular 6 and 4 person Lyft vehicles - which are of course a lot cheaper to use but a lot less fun!).
The drivers can take you anywhere on property - and for the same flat $20, they can take one passenger to one destination and other passengers (in the same party) to another destination. They'll even let you stop and do a short errand - not sure what the limits to this might be! And when you're ready to go back to your non-participating resort, you can have them pick you up and take you there.
If you tell bell services that you're not a guest of the BC/YC, Boardwalk, or WL, but want a Minnie Van activation code - they should tell you that's no problem. If the first person you ask tells you "no," then ask someone else, because they're mistaken. Ask to talk to the Bell captain!
Once the Minnie Van option has been activated, it will stay there for future trips. It's kind of addictive!