I did a search on Google about sphagnum moss in aquariums and didn't get much info. I did find someone who used sphagnum to anchor their plants in plant pots, so that would suggest to me that it isn't toxic or anything like that.
I suppose the thing would be is whether or not it would survive or just die. You could try a test by keeping some in a container, submerged on a window sill or something like that