As far as I know, the best you can do is something like this:https://www.warlight.net/Play?PreviewMap=38585
The red stripes are a combined object set to BonusLink_Spain. The yellow underneath is just a plain object, set to the other color on their flag, and with a 1 pixel yellow border. You could add the emblem of the flag there as well.
There are two problems. First, the entire area between doesn't all register, so there are gaps when you mouseover. The second problem is, it might not work with all flags, at least not without some creativity. The object you use as the actual bonus object needs to be the size of the entire flag.
Edited 1/1/2015 18:41:41