I agree with evryone else u need to take this design to a tattoo artist as a base idea and let him/her run with it and clean it up and i think it will be a cool tat.

For a tattoo on your lower back(Bum) make sure that the waistband of your pants is loose enough so that it doesn’t rub.
and try going commando for a while to stop the waist band on your underwear rubing it but apart from that treat it like any outher tattoo. Hope this helps.

Hi
after reading your thread i did a bit of surfing and came across this articale hope it is of help as it recomends a tattoo studio in london who specalises in white ink.
http://www.bme.com/tattoo/A60323/tatmyfma.html.