Would be great for modpacks where you can't easily provide the player with lava or another liquid fuel (without it being totally arbitrary) until well after they've built a smeltery. Also just useful in general. For some reason I thought this was already possible, planned pack progression around it, and was then sorely disappointed :p

Edit: Would feel consistent with the tank being usable under the Melter, too.

Extra note: maybe the Heater burns solid fuel faster when used in a Smeltery, to account for how much more ore can be smelted at once. If that's even possible.

I have no plans to add this feature. The Tinkers Construct smeltery code is pretty strict about requiring liquid fuels, and even if it were possible I feel it damages the balance and progression. Most of the reason for the melter is for packs where the smeltety comes later than you should be able to melt metals, and the alloy tank was added for the same reason.

I do provide a late game option for solid fuel: you can use the high oven to produce steam which can be used as a smeltety fuel.