That does not depend on murrine, but I guess it could be done with some widget matching in the gtkrc.
for example (this is not correct, just to give you the idea) widget_class "*Panel*GtkButton*".....

Hello there, thanks for the pointer! I'm pretty new to the whole gtk engine thing, but i've figured out exactly what I wanted to do. For people who wish to do something similar, this is what I added to the gtkrc file to only make the gnome main menu buttons (not the menu itself) rounded... This will however theme the other buttons in a similar way: style "murrine-gnomemainmenu"
{
engine "murrine"
{
roundness = 7
}
} widget_class "*Panel*MenuBar*" style "murrine-gnomemainmenu" Hope this helps anyone with similar issues
Cheers
Dan