In theory yes, but in the best cases you would have a very heavy load joined by stitches and not much more. Plus a few flattened tourists at the first breeze. An engineer could design a device gluing a inner core of hard wood or so, but we would need an expert on materials for that, and political will to allocate resources and do it fast.
The tree will regrow from the basis and grow fast for a while, but its health is doomed forever, will be also very vulnerable to cattle and wind (hanging forever from a hollowed trunk). nobody alive will see a 300 Yo healthy tree here anymore.
Acer pseudoplatanus has a semi-soft wood. That fact is relevant to investigate if the Satan's little tool needed help or not. Doing the same with an oak would be much more difficult for an adolescent.
My bet is that there was a team here, and the boy role was to be chewed by the police as a bait. Knowing it or not. The intellectual author could be in a different country by now.
> An engineer could design a device gluing a inner core of hard wood or so ...
You'd probably be better off to use an internal splint/pin of some sort.
Rough guess (with no calculations done), something along the lines of drilling a 30cm wide hole both ~half a metre upwards into the remaining trunk, and downwards into the stump, then fit a matching stainless steel rod to mount the tree back onto the stump.
To keep it from spinning on that rod (!), you'd probably use a second smaller pin (20cm x 40cm?) offset a bit.
No idea how to splice the upper and lower bark together though, such that there is adequate nutrient flow. :(
Stainless steel is rigid. We would need a material with some flexibility probably
Connecting vases is the easy part as long as is done fast, or the cut piece is covered and protected from became too dry and develop longitudinal cracks. Just sinking it in running, good quality, freshwater would help for weeks probably. Maybe even months.
First, all leaves and some branches must be removed to avoid fast dehydration. Both surfaces have to be polished to 1) assure a good contact (the chainsaw removes a few centimeter ring of the wood, so you can just put it in place and wait) and 2) achieve perfectly 180 degrees flat surfaces so all the forces are applied vertically and the trunk remains in the same place instead to slip to the right for example.
Then you need to (pressure?) wash the surface to remove any debris and unclog the vases.
next step would be to put both trunks in the exact position with a crane (should be easy part. The trunk section is not a regular circle). The idea would be trying to connect most possible vases at least in a side even if you don't achieve a 100% contact and the other side must end very displaced. As long as you can somehow assure that the weight has accurate support, this is all.
Survival is not guaranteed. no government has been done this before, but any politician succeeding on this would send a strong message, and probably became very popular overnight. Just trying would improve their image.
A tree is 'mostly dead', the inside doesn't do much, the outside is where all the action is and grafts are usually done with very young material so there will be plenty of support by the time it gets larger and heavier. The proportion of 'living:dead' material is much more amendable to grafting too. And it tends to be done with budding branches rather than a whole tree, in fact I haven't been able to find any reference of someone cutting down a tree and putting it back together, especially one of this size, the structural challenge alone makes it something that I do not think will work, especially not in that location with the tree taking the brunt of the wind load through that gap.