I've seen a bunch of articles on doing water for dios, but I can't recall ever seeing anything for lava.
I have a large scale dio planned. I will spare you the details, but it's Star Wars Episode III, the Mustafar duel.
I'm wondering if anyone has tried lava before.
I'm hoping to be able to have it lit from below to give it a glow, so the "lava" would have to be translucent and tinted orange.

Do you think that resin or other water product would work, could be painted and then lit from below?

Any, any thoughts would be much appreciated,
thank you.

You can add lights just below the water. Since the water is translucent, it will definitely glow.

I have a few ideas for you to try.
For the 'pillowy' round lava I would experiment with a 'spray' foam insulation. In the US it's called "Great Stuff". It's a foam that expands when you spray it. Use this stuff and some simple 'daming' material to get the size and basic shape. Once it drys you can cut it with a hobby knife to sculpt it.
If this doesn't give you the desired texture, it should get you close to the shape and you can use it for a base mold. Get this stuff in the shape you want, then make an RTV mold out of it. Then use either resin or plaster to pour in for the base.
Think about pouring the product in thin uneven layers to get varying thicknesses.
Then as mentioned put a light source underneed. Use a 'photo gel' colored orange to give it color.

I was thinking about bathroom caulking for a base shape material.

For the waterfall/liquid lava use a tinted resin "Water" product and pour it as if you were making a water fall. The light source will bring it to life.

Be careful with the light source and resin. The light can't be so hot that it softens the resin.

I must say this is a very challenging undertaking. Fire is one of the hardest things to model. Water is a challenge too and you're mixing the two.

It sounds like a good diorama- keep us posted.

Carving "Great Stuff" leaves you with voids that would need to be filled. (as it's a Open Cell Polyisocyanurate insulation) I Believe that there are several Closed cell ones on the market (they're listed as waterproof) Poly expanding foams........I'm just not sure of Brands / Names...........

Well I know GamesWorkshop cast one of their LOTR Balrogs in clear resin then lit it and painted the hard crust, so it can be succesful.

You can still produce a convincing lava flow by painting - a bright yello will work well - someone's tried it recently for a small scale mini over on Coolminiornot:
http://www.coolminiornot.com/forums/viewthread.php?tid=11700
which looks pretty good

Thank you VERY much, everyone, for your replies. Some very interesting insights added to what I was thinking already.
I will have to consult with my cohort and I'll get back to you on what we do with this.