Awww - free-ranging piggles are the most amazing thing ever! I have 2 boys - Toby & Barney, they are 2 in March. When I originally got them MrS thought they were absolutely horrible - 'skittish little rats without tails' were the words and they were to never, EVER come into the house. That was in May 2013 - by the October they had moved into the lounge, hutch n all. By the November they were free-ranging and if I ask MrS if he loves the pigs he replies ' no - I don't love the pigs, I worship the pigs' - and he really does!
So free-ranging came about one morning when we said 'wonder what would happen if we let them out'. We were totally won over by the investigating, zooming, popcorning little beasties. Then they crawled under MrS's big leather recliner chair and wouldn't come out. At this point we thought 'Oh god...the carpet'. We now have strategically placed puppy pads and hay boxes. Piggies do like to pee in secluded places so we lined under the chair with puppy pads (its like one giant hidey) and they spend pretty much all morning under there. I then have a cardboard box lined with puppy pads and full of hay half way across the lounge and a pet carrier again with puppy pad and hay at the far end of the lounge so there is always somewhere for them to go. And (touch wood) to date we haven't had any accidents (that we have been aware of). We do have occasional little poops fall out but that's nothing a hand vac cant deal with. Personally I wouldn't advise getting stressed by trying to train them (but that's just because it didn't work for mine), I would just watch if they have favourite places to sit and stick a puppy pad or hay filled box down.
If your piggies do free-range you will have an amazing relationship with them - you get to know them so much more when they can come running over to you and try climbing up your legs trying to bully you into giving them your salad off your dinner plate. The laughter when they are zooming around the furniture and you get a real sense of their personality. I couldn't ever go back to keeping piggies caged. Saying that - if we are both at work they stay in their hutch & run and at night time but if either of us is off then the pin comes out of the run at 8am and they put themselves to bed at 10pm. We have a lovely little routine going and I love standing the kitchen cooking dinner with pair of them in the doorway shouting at me for veggies!
However, if you had told me that I was going to live in a field one day I would not have believed you. There is hay EVERYWHERE - we no longer call the dyson a hoover - its a baling machine now!
We love it - give it a try - if it doesn't work out you can go back to keeping them caged.