Is anal douching safe?

Depends. Anal douching would be similar to daily enema use. Safety depends on what you are using to rinse with as well as how it is applied. Some instruments can cause anal / rectal irritation and even ulcers. So be careful what you use and how you use it. Recommend you consult with your doctor first.
I would think so. Not much more different than an enema. I would caution not to do it tooo often. We don't do large scale double blind controlled studies to discover the side effects of that sort of thing for obvious reasons.