if you have direct access to the machine you might be able to use one of the many recovery CD's available on the internet, if you dont have physical access to the machine then i am not sure how it is possible
if you did an install on top of debian you should have the non priveleged account which has sudo access which should be able to sudo to change the root password
rule number 1 in server admin NEVER FORGET the root password and ideally but currently not possible in proxmox is never use the root account for normal usage
that said a proxmox developer may be able to assist you privately, i aggree that i wouldnt want possible methods to crack the security of proxmox being made public