Trizen, the same person that produced Obmenu-generator, has also produced a similar dynamical menu that rides in the system tray rather than on the panel. A Menutray is pictured on the right as well as in the link just referenced. To install download, unzip and follow the directions. Dependencies can be installed as follows:
sudo apt-get install cpanminus gexec libcpan-distnameinfo-perl libcpan-meta-check-perl libfile-pushd-perl liblocal-lib-perl libmodule-cpanfile-perl libparse-pmfile-perl libstring-shellquote-perl libdata-dump-perl libgtk3-perl ,
which, except for libgtk3, are approximately the same as required for Obmenu-generator. You only need libgtk3 if you want to use the gtk3 version of the menu. The command to launch a gtk3 menu with icons is
menutray -i -gtk3 .
The menu offers 11 dynamical categories: utility, development, education, game, audiovideo, network, office, other, settings, system. Favorites or Quick Launchers can be added or removed by editing one line in the schema file. Similarly, new categories can be easily added. It is a very nice menu - lightweight, clean, easy to install and edit, and frees up the space a regular panel menu would take.
The picture on the right has a transparent fbpanel on the left extending 70% across the bottom edge. The box at the bottom of the picture is an openbox dock placed in the right 30% of the bottom edge, but it can be positioned anywhere on the screen using the Openbox floating dock setting. Inside the dock is a Stalone Tray, which contains the regular tray icons as well as the menutray icon.