Most of the beech in your area aren't bonsai ;-) Hollow trunked deciduous bonsai aren't distance runners. Hollow trunk deciduous bonsai don't last a very long time. Been there--done it with a few trees...dealt with the issues. Dealing with the issue now on a century old collected oak.
FWIW, a Japanese-trained bonsai instructor told me the same a while back. He said there are practically no deciduous trees with hollow trunks in Japan not because they break the rules, but because any deciduous tree with deadwood in its interior is understood to have a death sentence. He freaked out over a friend's hollowed out elm, saying the tree would be a shadow of itself, or dead within 15 years. He said the deadwood opens the interior of the tree up to all kinds of issues, from insect infestation and damage, to drying out and extreme temperatures. Rot is the primary enemy though. It is a slow killer.
No amount of filler, cement, etc. can stop rot. In fact, such treatment can ACCELERATE the issue, by sealing off moisture that gets behind it. Such treatment also gives owners a false sense of security. Many owners don't do the due diligence in drying the wood out and preparing it for sealants, fillers etc. Some treatment includes drying out the interior of the nebari BELOW the soil level. I would be surprised if this tree didn't have rot down below the soil level.
Not saying this tree shouldn't be a bonsai, but don't fall in love with a potentially fatal wound. Know the problems going in and work to mitigate them. Regular Lime sulfur application can help more than filler, as can removing rotted wood thoroughly--which means at next repot, having a look at the trunk from the underneath to see if you've got rot there. Work to get in and actually get an eyeball on the bottom of the trunk--which could mean clearing away significant rooting...Deadwood--like the damage on this tree--wicks water into the interior wood, where it drains downward. It never dries out on the interior. The bottoms of hollowed out trunk pools water that drains down in the remaining heartwood under the soil...