like somebody else said I can't remember who it was, but if the horse is perfectly fine with you picking up their hind feet, then brushing their tail shouldn't be a problem, unless you don't trust that horse.
I stand behind my horse but not directly because I know he can't see there, but off to the side a little and I wear brightly colored clothes so im pretty easy to see, I also sing when I groom him so he knows im back there, and it relaxes him to hear the calm tones in my voice. Of course my horse when I first got him didn't like his tail brushed let alone his butt touched lol
I just worked with him and he's totally fine with it now.
Our horses know our secrets; we braid our tears into their manes and whisper our hopes into their ears.
Truly War Paint-Arabian,QH,TB,Paint,Barb-Diablo