How to combine fill and pattern in a pgfplot bar plot?

You can use a postaction to draw the pattern after the yellow background.

\documentclass[12pt]{article}
\usepackage{pgfplots}
\usepackage{tikz}
\usetikzlibrary{patterns}


\begin{document}
\begin{tikzpicture}
\begin{axis}[ybar, samples=10, enlarge y limits=upper, ymin=0]
\addplot +[
    black,
    fill=yellow,
    postaction={
        pattern=north east lines
    }
]{rnd};
\end{axis}

\end{tikzpicture}
\end{document}

---

In older versions of PGFfplots, you needed to use every path to apply the pattern.

In order to apply the postaction, you can use every path/.style={postaction={...}}. This requires that you clear the postaction, e.g. using the approach given in Applying a postaction to every path in TikZ, as otherwise you get an infinite recursion.

\documentclass[12pt]{article}
\usepackage{pgfplots}
\usepackage{tikz}
\usetikzlibrary{patterns}

\makeatletter
\tikzset{nomorepostaction/.code=\let\tikz@postactions\pgfutil@empty}
\makeatother

\begin{document}
\begin{tikzpicture}
\begin{axis}[ybar, samples=10]
\addplot +[
    black,
    fill=yellow,
    every path/.style={
        postaction={
            nomorepostaction,
            pattern=north east lines
        }
    }
]{rnd};
\end{axis}

\end{tikzpicture}
\end{document}

I think the answer is yes: by two separate path instructions with the same path: one without pattern but with fill color and one with the chosen pattern.

In terms of the \addplot command of pgfplots: two separate \addplot commands, one with \addplot[fill,forget plot,...] and one with the pattern. The forget plot tells pgfplots to avoid legend entries for the plot.

If this appears to be unsuitable, you may want to look into decorations - perhaps they support such a thing.