So it sounds like we wont have Grenzers models for a while, so what do you think will make good proxy models?
Off the top of my head, either of the Heckler models could do nicely, maybe with a little conversion. The Grenzer has that very Tunguska style long coat thing. Maybe a Prowler. Gecko Pilot. Intruder, as long as you weren't running an actual Intruder. Wildcat Spitfire model.
Whatever you aren't using at all in your army? If you're playing Tunguska sectorial, Moderators or Aguaciles? If you're reaching a little farther afield, Guilang have helmets and long coats.
The Sineater would make an excellent Grenzer. One of the things that I like about this game is that the HMG is so similar in size to rifles, proxying isn't a big deal visually.
Funny enough, that's actually one of my pet peeves, it's too easy to mistake a HMG for a Rifle if you don't know what that faction's HMG looks like these days. HMGs used to be really huge, far bigger than the rifles. As it is now, I still use the old Nomad HMG on troops, but I mod the heck out of the Yu Jing HMG (give it a big ammo box and a belt feed).