Thanks to all who replied -
I tried it today, and it worked.
Maybe it's just a question of using the right
IP address. (My vonage phone doesn't accept https :-)
I now have easy access to my m0n0wall from the internet.
In case others are contemplating the same,
I now have my m0n0wall in parrallel with my pebble.
I can work on this until it is ready to replace the pebble.