Hi - great intentions, but I really don't think it's a good idea. It's not just about temperature (and I wonder if ME nights can get pretty cold on odd days even in summer), but the whole picture of general incompatibility. Tropical plants do well in their own environment, from soil make-up, to insects, to rainfall - how much, how cold and when, to wind, etc. and I'd be much more inclined to put it out, but in a pot and not the ground. You may be trying to fatten it, but a few months won't be enough to do what you want anyway, certainly not enough e.g. for native trees (3 yrs is considered the minimum to make a difference) and the adjustment time alone for your tree, only to be followed relatively soon after by digging up (and bringing with it who knows what) will not be helpful. And besides, Ficus in fact do better when slightly constricted rather than given infinite ground, though that's not going to be so in places they're native to, that are hot and humid all year long.

Also - ME doesn't have enough hours of sun to help Ficus do well outside full time, but at least if it gets enough through the rest of the year (indoors) it'll have a better chance.

If you are you on the southern coast of Maine and your night time temps are over 45 degrees You can try but watch your night time temps. I live on Cape Ann and my Ficus has been out for a week with night temps over 45 degrees F but I agree with larke,leave it in the pot and in the sun. Don't be surprised if you have leaf drop and be ready to bring it in If a low temp is predicted.

This is the thing its kind of a experemental ficus. It is in the process of beeing fused it was originaly 5 skinny little b.s trees in one pot. Probaly from home depot or something. I like the the leaves. So i decided to try and fuse them. I started the project december 25. of 09. I took off the string the other day to check the progress. it had begun. so i figure what the heck! The only reason why i wanted to stick them in the ground for the summer was to speed up the process. they seem to pretty tough. and I thought the other benificial to exposeing them to the elements of maine, might be making a more hearty specimen..

Or a deader one... it's working hard enough at fusing, why take the chance when the rewards for one short season are not going to be worth it?

There's a reason why tropicals and sub-tropicals don't grow in Maine. You are also playing with the 'weak-sister' of the ficus family in your project. From most of your posts it seems you are interested mostly in growing tropicals, I will suggest that you build a greenhouse before going too much deeper into Bonsai.
One thing you can do to the ficus is move it into a large shallow training box or pot.

Bob