Your equation works but it has no meaning. You can't assume v>c means negative time when you are talking about your equation, it represents nothing in physical reality:
It is equivalent to
f(x) = \sqrt{1-\frac{x^2}{k}}
Why would someone consider that function to have anything to do with reality, until he observes reality to follow that pattern? If they would, why not
f(x) = \frac{e^xsin(x)}{(x^3)(x^2-2x+e^xln(.47sin(x)))}? I could say when ln(.47sin(x)) is larger than e^x then such and such happens, but there's no reason to say that the pattern of increase of those two functions will describe how something works.
You model equations to fit the world, not model the world to fit equations. I understnad your intent wasnt to model anything physical, but when you take a relatively (no pun intended) infamous equation and start manipulating it to do something you want, without changing the variables, you are implying that you are attempting to change that model (for that specific problem). If you had instead used
f(x) = \sqrt{1-\frac{x^2}{k}}
instead, you wouldn't have any arguments as to the validity of the equation for modeling purposes.