I know you are wondering about alternatives, so I don't want to de-rail, but I have spent a few hours looking into this and I think it is feasible for GW/Forgeworld to do it. I also believe they can do it on a Mumak base.
Some people have tried to estimate the size of smaug and used the coins and Bilbo from other parts of the movie for scale along with playing around with photoshop and came up with the result that Smaug would be 3/4 of the length of a soccer field.
For the life of me, I can't understand why they don't just compare Smaugs head to Bilbo when they are right next to each other as its the same scale.
It would take roughly 1.5 to 2 Bilbo's to equal Smaug's head height and roughly 1.5 times that again for head length (snout to spine), or roughly 4 Bilbo's long. Bilbo is a fraction under 1 inch tall in both LOTR/Hobbit minis (i thought there was disparity, but it must only be for dwarves). So Smaug's head would be about 4" long and 2" high.
The main challenge of course is the length. It would be exceptionally long to do a straight/flying version, easily 10-12" long, but of course, that is not really a good idea unless GW opts to put him on a flying, mumak sized base. This actually is a smart choice from a gaming perspective, as he would be taller than generic troops with spears and would allow them to sculpt it without having to worry about the model extending inches over the base or making another master base for just one model. Anyway, I don't think they they will do this, rather they would just curl the tail back to the front of the base or wrap it around some debris.
Smaug looks very big when he rears up to show Bilbo is size, but it's important to remember he actually has quite a thin body (or he would have trouble either walking or flying) so there actually isn't all that much to put behind the torso. You could actually give him a 6" torso and have another few inches to get the hind legs and the tail on the base if you bend it a little. When you consider that you can have the entire head and neck overhanging the base (as they do with other models), this gives a lot of room to play with.
There is a small feature on Smaug by Weta (link below) which has an awesome CGI model near the beginning. To me it shows just enough of everything to prove it is feasible - that you can have a dynamic pose, overhanging head, wings near the body and the position of the hind legs. The only thing left to worry about is the tail. In this model, the tail is mostly straight with a bit of vertical arch, so a bit of fiddling and you can have the tail overhanging the base, but out of the way, or curled around. The model doesn't have to be rearing, which is very important towards having a feasible model to sculpt, cast and distribute to customers without expecting great difficulty in assembly. I wouldn't be surprised if GW was using that same model.
https://www.youtube.com/watch?v=Pvr7DSEHcic