In this General Boundary Waters / Quetico Discussion forum, in another topic titled "first time Quetico trip" the following advice is given regarding entry into Quetico:
"Paddle in from Horse on a day permit through BWCA. You can make canada (moose bay) in 5-6 hrs. RABC...lets you cross into canada without going through customs.  It is a canadian document that pre-clears you through canadian ports-of-entry"

Is this correct? Have the regulations changed?  I have always gone through the Prairie Portage ranger station.  Can I just enter Quetico whenever and wherever I want as long as I have a RABC?

Almost correct.

Whenever, wherever you have paid and permitted for.  The remainder of that thread talks about going up to PP the day prior.

Pre and post season you dont have to see the ranger.  Just get your fees in the box at PP.

Just to clarify things and see if I understand.
For example, if I have a reservation(?) to enter at Prairie Portage on July 4, I need to get a tow up Moose Lake to the ranger station at PP on July 4 and get my permit/paperwork and once I have that, get a tow back down Moose, jump in the car and drive to Mudro/Horse Lake in the BWCA and paddle through the BWCA and enter the Q from that direction.

Did you mean backtrack and start at Mudro (leading you to Horse Lake)?

You'd be going thru Moose to get to PP anyway.

I doubt if that would be legal, but you could ask the Canucks for their take on it. 'Don't ask, don't tell' doesn't apply to your question...

Rocky, I think that would be OK if your the entry point on the permit was where you would be entering the park after your tow back down. I'd check though.