Is it possible to have an equation in align which crosses alignment boundaries?

You can set the "longer explanation" inside a zero-width box and let it extend across the align alignment points:

\documentclass{article}

\usepackage{amsmath}

\setlength{\textwidth}{30em}

\begin{document}

\begin{align}
  & x = a + b + c + d + e + f + g + h + i + j, & \text{some condition} \\
  & y = a + b + c + d + e + f + g + h + i + j, & \\
  & & \makebox[0pt][r]{some much longer condition} \nonumber
\end{align}

\end{document}

---

Vertical alignment of the tag (2) is possible:

\begin{align}
  & x = a + b + c + d + e + f + g + h + i + j, & \text{some condition} \\
  & \begin{array}{@{} l}
    y = a + b + c + d + e + f + g + h + i + j, \\
    \mathstrut
  \end{array} & 
  \begin{array}{ r @{}}
    \mathstrut \\
    \makebox[0pt][r]{some much longer condition}        
  \end{array}
\end{align}