Triatic Stay

[Brion Toss]

Yup, the title lists those vessels that can legitimately boast having triatics; people with ketches, looking for an excuse for saltiness, have long tried to pretend that they posessed something sexier than a springstay. The same people, as well as lots with only one mast, have tried to claim "whisker stays", when, without spreaders (whiskers) they actually just have plain old bowsprit shrouds. I may be fighting a losing battle here (don't get me started on "stays'l stays), but I am of the belief that, where possible, different things should have different names.
Anyway, to the questions. First, HM would be perfect for a springstay, much preferable to wire; the loads are trivial, there's no danger of chafe, and the reduction in weight aloft will be significant. Spectra would be okay, though for long-term Vectran, covered, would be more durable and wouldn't ever creep.
My favorite configuration involves installing a turning block in the top of the mast, then running the springstay inside, like a halyard. You can even belay it like one, though if you do you'll want an exaggerated failsafe setup, to prevent its being cast off accidentally. But it is good to be able to adjust it, particularly if you also have an adjuster on the main backstay. Which you should. But you could also just lash the springstay at the masthead.
As I understand it, the springstay makes a good antenna, given its height and angle, but it's best if the tuner is up on the mast. Anyway, an insulated backstay works just fine.
The most important question you had is about whether you could move the attachment point of the springstay, and the answer is, of course, "it depends". There are lots of ways to configure a mizzen's rigging, and the attachment point of the springstay must, must, must be integral to the design; you can't just move things around without considering the system as a whole. You might want to look at the chapter on mizzens in the Apprentice, to get an idea of how things can work well, as well as badly. The placement has to do with geometry, not what the masts are made of.
Fair leads,
Brion Toss
PS,
If you haven't seen the Falls of Clyde photo gallery on our home page, I urge you to take a look. Many pictures, giving a good feel for the scope and scale of the project.