They only manufacture parts for a certain amount of time them when the stock runs out that's it. Same with any manufacturer.
1200 pistons are .5 mm larger than 1100s so it could be bored to fit but whether the rest of the piston would match I don't know. I can't find specs on the wrist pin diameter and piston height from the pin center.
I use this site in uk its www.davidsilverspares.co.uk they post to usa and have a .75mm ring set for your gl1100. Give it a look. I agree with over bore in that i have been informed they cannot be bored out. Good luck
Im sure some one has a piston that will work in your bike that was not made for it. the trick is getting the piston height & pin size correct. but you may have to cut valve reliefs in it (shone how in other blogs) google found me 2 kawasake trany bearrings that worked in my gl1000 $40.00 & 74 honda civic full ring sets also.$45.00-allso check hed gosket if it will recieve over bore.