These scripts add and remove IPv4 forwarding and NAT from a Linux bridge while still allowing it to be managed by Proxmox.

vmbr1 up and down

Place each file in /etc/network/if-up.d/ or /etc/network/if-down.d/ as appropriate, mark as executable, and name whatever you want.

vmbr1-down

#!/bin/bash -e
# /etc/network/if-down.d/vmbr1
if [[ $IFACE == "vmbr1" ]]; then
  echo 0 > /proc/sys/net/ipv4/ip_forward
  iptables -t nat -D POSTROUTING -s '10.0.0.0/8' -o vmbr0 -j MASQUERADE
fi

vmbr1-up

#!/bin/bash -e
# /etc/network/if-up.d/vmbr1
if [[ $IFACE == "vmbr1" ]]; then
  echo 1 > /proc/sys/net/ipv4/ip_forward
  iptables -t nat -A POSTROUTING -s '10.0.0.0/8' -o vmbr0 -j MASQUERADE
fi