If Xcompmgr is used solely to provide tint2 with transparency effects it can be run at boot by changing the autostart section in ~/.config/openbox/autostart to this:

Launch Xcomppmgr and tint2 with openbox

if which tint2 >/dev/null 2>&1; then
(sleep 2 && xcompmgr) &
(sleep 2 && tint2) &
fi

Other (better) ways to make Xcompmgr run at startup are discussed in the Openbox article.
Fullscreen/Overlay

To force tint2 to stay on top of the application (overlay), you need to set the panel_layer option appropriately. This can be helpful when you switch from a fullscreen window to a normal application using Alt-Tab. There is a discussion on this at Crunchbang Forum

#Panel
panel_layer = top
strut_policy = follow_size