You fill a watermelon the same way no matter what kind of liqueur you use.
Just take the following recipes in the links below and substitute with liqueur
of your choice. Start about 48 hours before serving.
see links:
[Kent Garber] Re: alcohol + watermelon------------------------------------------------------------
This web site features Tequila Spiked Watermelon Soup
http://recipe-appetizer.com/31/167366.shtml