Did anyone find a way to implement backup&restore using a Veeam backup repository? Or is it possible to install Kasten K10 in the main cluster?

Not currently supported, could you create an issue for https://github.com/harvester/harvester/issues/new/choose?

talking to veeam they support kubevirt so it would just be a case of installing k10 into the harvester cluster to backup the vm's which harvester should be supporting deployments in harvester clusters soon but I haven't tested if it works already https://docs.kasten.io/latest/usage/openshift_virtualization.html?highlight=kubevirt