If you want an honest answer, it
is against the rules to park in any resort parking lot to visit a theme park. As you enter BCV, you will see a great big sign to that effect.
As others have noted, however, there is a huge difference between what Disney
says and what they actually enforce. (See the exhaustive smoking, pool-hopping, occupancy, dress code, dining plan, and other debates for examples.)
If your question is whether there is any
enforcement mechanism, I didn't see any when we were at BCV last year.
I believe the Swan and Dolphin have gone to
paid self-parking because of abuse. They are not Disney properties, so that policy doesn't necessarily mean
DVC will start charging for self-parking some day.
Disney could start charging for parking at all their locations, because there is no question there is an issue with people using free parking and Disney transportation to avoid paying the theme park parking fee. We hear it all the time here because of people parking at DTD and using Disney buses -- and then jamming the SSR buses to get back to their cars. I don't think Disney would do that based on
resort parking, but if the problem gets so severe at DTD that paying customers can't find parking because of freeloaders, they might.
If you have friends visiting you at any DVC
resort, I believe it is a simple matter to make arrangements for their parking (without paying anything) at the front desk. We've never had any problem doing that, no matter where we stayed.
And finally... If the walking-to-Epcot ease is truly the issue, it's a simple matter for your guests to use the valet service. That's a LOT more convenient than self-parking and noone could complain about anyone breaking rules to save $10.