hrule into tikz circle node

You could just draw a longer [h]rule and clip the node:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes}
\def\clap#1{\hbox to 0pt{\hss#1\hss}}
\begin{document}
\begin{tikzpicture}[auto, node distance=3cm,
    styleshess/.style={ellipse, draw, align=center,clip},
]
    \node[styleshess] (2) {NODE2\\\clap{\rule{\linewidth}{.4pt}}\\ long name \\ foo \\ bar \\ baz};
\end{tikzpicture}
\end{document}

You can play around with that line as with any other \rule to modify positioning or width. The \clap makes sure that the (imaginary) length of that line does not blow up the size of the node. The clip in the style definition makes sure that only the part of the line that falls within the node is actually drawn.

Something of that sort?

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{shapes,fit,positioning}

\begin{document}
    \begin{tikzpicture}[auto, node distance=3cm,
        style1/.style={ellipse split, draw, align=center},
        style2/.style={ellipse, draw, , align=center},
        lower elli/.style={align=center},
        fit elli/.style={ellipse,draw,inner sep=0pt,
        path picture={\draw ([xshift=-1cm]#1.south west)-- 
        ([xshift=1cm]#1.south east);}}
    ]

        \node[style1] (1) {NODE1 \nodepart{lower} long name \\ foo \\ bar \\ baz};

        \node[style2] (2) [right=1cm of 1] {NODE2 \\ long name \\ foo \\ bar \\ baz};

        \node (3a) [right=3.5cm of 2.north,anchor=north] {NODE3};
        \node[lower elli] (3b)[below=0pt of 3a] {long name \\ foo \\ bar \\ baz};
        \node[fit elli=3a,fit=(3a) (3b)]{};
    \end{tikzpicture}
\end{document}