is it possible to create a seperate user, and force ESXi to use that account for NFS access to NFS shares?
It is not possible to do this for ESXi. NFS access requires the use of the root user (UID 0) to access NFS shares and you have to disable root_squash.
For ESX there is "experimental support" for delegating NFS access to a separate account, as per this VMware document. That's VMware code for saying the functionality is there but if you run it on production systems nobody is going to help you if anything goes wrong.