Yes - it´s possible.
I am testing around some days with lmtp-delivery in PMG.
in my environment there are two PMG als mx servers and behind there will be an dovecot server.
The pmg should deliver with lmtp and dynamical address verification.
For testing purpose you only have two change the "smtp"-keyword in /etc/postfix/main.cf of the "relay_transport" option to "lmtp".
I have placed a enhancement request in Bugzilla:
https://bugzilla.proxmox.com/show_bug.cgi?id=2438
Please feel free to look there for some more informations.