I think the design I have now is thus:
There will be 4 Dragon Heads, each the same size, equidistant from the four corners of the main tower like so.
O---D---O
l............l
l............l
l............l
D1.........D
l............l
l............l
l............l
O---D---O
D = Dragon Heads
D1 = Dragon Head with drawbridge underneath.
O = Corners of tower
- and l = walls of tower.
This means one will be above the entrance drawbridge. It will be made of magma safe materials, and hooked to the same lever. Once pulled, the drawbridge rises, floodgates rise BEHIND the drawbridge to prevent any magma from entering the tower in any way. Floodgates in the dragon's mouth itself will then lower, providing a nice (hopefully) stream of magma, sealing my castle shut securely.
Pull the lever again, and the floodgates rise, stop the magma flow, and the drawbridge lowers, with its accompanying floodgates lowering as well, allowing caravans inside, as well as immigrants.
For magma storage, I hope to do some math and figure how much magma I'd need to fill a cistern at the top of the tower as well as the moat surrounding the structure itself. Not just 1/7, but a full 7/7 height in all tiles. This way, any extra magma that sneaks into the system will be the magma dropping back into the moat at that time.
The magma will be collected at 4 points in the moat, meet in an area of pumps in the center of one of the tower's basement levels, and be sent up a row of pumps that make up the fortress's "spine", with water also being ferried as well. It will empty in the cistern, where hopefully fluid dynamics takes care of the rest, and a level of equilibrium is found, allowing both the moat and cistern to remain at relatively stable levels, without one or the other overflowing.
Hopefully that well o' text made sense to you guys