Ahhh, that HSF. I'm horrible with abbreviations. No, I'm not planning on making HSF solid obsidian.
My plan at present is to create a 20 z-level obsidian fortress within the lava of the volcano at my embark site. Not only will this be an interesting defense, it will make for some definite fun at some point when something goes wrong. Not that I won't take every precaution to stave off said fun, which will make it all the more fun when the fun happens.
The idea is a lava moat... only rather than encircling the fort, it entirely envelopes it! Well, except for the entrance bridge/tunnel/tubes, which will also serve as the means of suspending the fortress in the lava. I think I may use windmills to power the machinery within the fortress, if I can acquire the necessary number of nether cap logs for a submerged power transfer system. (It would use a set of gear assemblies and a horizontal axle submerged within a short channel of lava, with un-submerged gear assemblies sitting atop the submerged ones, followed by a second similar setup that uses water instead of lava, to ensure that nothing can enter the fortress through the power transfer tunnel unless it can fly, swim through lava, and swim through water).
I will try to incorporate a system of pumps that can flood or drain every major horizontal traffic passageway with lava, and construct all of the doors from these passages to other sections of the fortress on the roof- ie, the dwarves will have their rooms, workshops, etc. atop the passageways. In the event of a catastrophic breach of the outer defenses, the dwarves would retreat into these upper rooms (via burrow designations) and a lever would be pulled to flood all the passageways with lava, ensuring that either the dwarves will be safe from harm in their rooms while the enemy is dealt with by the military, they will be trapped alone and eventually starve when the military is slaughtered, or they will be trapped and each forced to face the horribly powerful magma-safe foe when it finds them cowering in their rooms. Fun.
Of course, this is what I would -like- to do. I know it is -possible- to do, but it may take a ridiculously large amount of time to accomplish.