First, whether or not it is commonplace, there might be some danger in using an adorned "P" for mirabolic, since it suggests "parabolic". And, indeed, modulo the center, the mirabolic *is* parabolic, so maybe it doesn't matter too much.

Then, yes, for any parabolic there is a notion of Siegel set: for a choice of split torus and positive roots, the positive simple roots *not* among the root spaces appearing in the Levi component (or vice-versa, obviously there's a bijection between subsets of roots and their complements) gives inequalities specifying the corresponding Siegel set. That is, for positive roots $\alpha$ with rootspaces {\it not} in the Levi, the collection of inequalities $|\alpha(a)|\ge y_o>0$, together with the corresponding unipotent radical coordinates lying in a fixed compact, specify a Siegel set.

Depending on what is intended by "cuspidal" on that quotient, such functions may or may not be rapidly decreasing... and the notion of "rapidly decreasing" might need corresponding clarification.

E.g., it is true that a ${\mathfrak z}$-finite and $K$-finite function all whose constant terms (along a connected family of parabolics) vanish is of rapid decay _in_the_directions_ where the corresponding positive simple roots go to $+\infty$. That is, the functions need not be automorphic forms on $G_{\mathbb A}$. Indeed, that point is necessary in the general setting-up of Eisenstein series and automorphic spectral theory.