\hfill in math mode

You can do it, but not with \hfill, because of how align works. Here's a way:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\pushright}[1]{\ifmeasuring@#1\else\omit\hfill$\displaystyle#1$\fi\ignorespaces}
\newcommand{\pushleft}[1]{\ifmeasuring@#1\else\omit$\displaystyle#1$\hfill\fi\ignorespaces}
\makeatother

\begin{document}
\begin{align*}
    a + b + c &= a + b + c + d + e + f + g + h + i \\
    & \pushright{\text{(foo)}}
\end{align*}


\begin{align*}
    a + b + c + d + e + f + g + h + i &= a + b + c \\
    \pushleft{\text{(foo)}} &
\end{align*}

\end{document}

A slightly different implementation allows you to use \hfill where you want:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\specialcell}[1]{\ifmeasuring@#1\else\omit$\displaystyle#1$\ignorespaces\fi}
\makeatother

\begin{document}
\begin{align*}
    a + b + c &= a + b + c + d + e + f + g + h + i \\
    & \specialcell{\hfill\text{(foo)}}
\end{align*}


\begin{align*}
    a + b + c + d + e + f + g + h + i &= a + b + c \\
    \specialcell{\text{(foo)}\hfill} \\
    \specialcell{\hfill\text{(foo)}\hfill}
\end{align*}

\end{document}

I'd recommend against using the simple \omit.

In align you can use \omit at the start of cells:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
    a + b + c &= a + b + c + d + e + f + g + h + i \\        
              &\omit\hfill foo\\        
             a&=b\\        
   \omit text & =c \\
   \omit\hfill text&=d
\end{align*}
\end{document}

The most obvious answer is \hfill as demonstrated by this plain TeX file

$a\hfill b$

\bye