I am a die-hard fan of Mutsus (also called Crispins or Senchus,) but they're hard to come by outside of a farmer's market. I also highly recommend the use of a foodsaver vacuum sealer for freezing pie fillings: if you fill the bag and put it in your pie dish (you should make sure you use a standard size) and then vacuum and seal, it holds its shape. Last year I used ziploc bags; they didn't present any problems but the apples weren't held together and it wasn't as pretty.
My recipe is loosely adapted from the Farm Journal's complete pie cookbook: I cut up about 8 medium-to-large apples, (make sure you sprinkle them with something to prevent browning - I happen to have citric acid, so I used that.) Mix together 1/4 cup of brown sugar, 1/2 cup regular sugar (sometimes I use vanilla sugar) 3 tbsp of instant tapioca or flour (I'm not so happy with the tapioca and am going back to flour) 1/4 tsp of cinnamon and 1/8 tsp of allspice (I like my pie to taste mostly of apple, so I season accordingly) Sprinkle a heavy layer on the bottom of the bag you're filling, and then toss the apples with the remainder before you put them in the bag and seal them. When you're ready to bake, line your pie shell with pastry, dump in your frozen apples (the sugar will probably keep them from holding together like a brick, so be a bit careful) and top with remaining pastry. Bake at 450 for about an hour.
I do get a bit of seepage, but also a benefit: because moisture gets released when they're frozen, you don't have the usual problems with your top crust sagging. If you have enough thickener (it does depend a bit on the type of fruit) you shouldn't have too much trouble. Prior to discovering Mutsus I used Granny Smiths, but I wasn't as thrilled with them. BR suggested a combo of Winesap and Northern Spy, but I'm not sure where he sources them.
FWIW, on her website, Paula Hainey suggests dusting the inside of your pie crust with the crumbs and flour left from rolling out your pie dough. Makes sense: essentially piecrust crumbs are a slightly wet
beurre manié.