I'd define a new environment, to which you give the desired label as argument.
\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newenvironment{varsubequations}[1]
{%
\addtocounter{equation}{-1}%
\begin{subequations}
\renewcommand{\theparentequation}{#1}%
\def\@currentlabel{#1}%
}
{%
\end{subequations}\ignorespacesafterend
}
\makeatother
\begin{document}
An equation
\begin{equation}
1=1
\end{equation}
Some subequations
\begin{varsubequations}{P1}
\label{eq:Parent}
\begin{align}
a& \label{eq:1}\\
b& \label{eq:2}
\end{align}
\end{varsubequations}
Another equation
\begin{equation}
2=2
\end{equation}
Now the references: \eqref{eq:Parent}, \eqref{eq:1}, \eqref{eq:2}.
\end{document}
