If you truly do not have access to the back, then don't mount anything that can take a high load there.
To fill the hole, you can probably get away with just mixing up some very thick epoxy and carefully laying it in without pressing too much out the back. Alternatively, you can cut a disc from cardboard that is a bit larger than the hole. Make a very small hole in the center of the discs and run a piece of string through it. Tie a large enough knot in the string to prevent it from being pulled back out through the hole in the cardboard disc. Fold the cardboard neatly in a way that will allow you to insert it through the hole. When you pull back on the string, It should expand the cardboard and prevent run off through the back off the hole. Tie the string off tightly and fill the hole with thickened epoxy (peanut butter consistency).
To fill the hole, you can probably get away with just mixing up some very thick epoxy and carefully laying it in without pressing too much out the back. Alternatively, you can cut a disc from cardboard that is a bit larger than the hole. Make a very small hole in the center of the discs and run a piece of string through it. Tie a large enough knot in the string to prevent it from being pulled back out through the hole in the cardboard disc. Fold the cardboard neatly in a way that will allow you to insert it through the hole. When you pull back on the string, It should expand the cardboard and prevent run off through the back off the hole. Tie the string off tightly and fill the hole with thickened epoxy (peanut butter consistency).