SNAT with --persistent

I am trying to do source NAT-ing where there is a pool of translation addresses and any given internal client gets the same public IP each time it connects out. In pure iptables this is easy to do: just add the --persistent option to the SNAT line. I just don’t know how to do it on VyOS (version 1.1.3).

    Our config.boot currently has a section like this:

nat {
source {
rule 1 {
outbound-interface eth0
source {
translation {
address x.x.x.1-x.x.x.16

This becomes the following in the iptables nat table:

-A POSTROUTING -s -o eth0 -m comment --comment SRC-NAT-1 -j SNAT --to-source x.x.x.1-x.x.x.16

What I want is this:

-A POSTROUTING -s -o eth0 -m comment --comment SRC-NAT-1 -j SNAT --to-source x.x.x.1-x.x.x.16 --persistent

    I have actually tested this config by making the change directly in iptables and it works. It does exactly what we want. Unfortunately I have not found anything in the VyOS documentation about a "persistent" keyword for source NAT. Is there a way to add persistence for an snat rule via the VyOS configuration?