I'm nearly 14 weeks and sleep on my tummy every night. My midwife said it's safe to do this and said there will come a time when I won't be able to because it will become too uncomfortable. So if I do it evey night I'm sure once for you to get a back rub will be ok.
Sit up and hug a pillow leaning forward...if that makes sense...if your not comfortable laying on your tummy. I use to do that when I was 7/8 months pregnant when hubby would rub my back. :0)