If one is small enough that they are unregulated (or perhaps no one is looking at your effluent), and the chemical costs are insignificant, then it's probably not an issue.
I don't have any useful knowledge about the effect of these. Aside from what you (PE) say, about the only thing I'd have to go on would be Kodak literature describing a two-stage fixing system. In my mind, this is an endorsement of the system. (At least when Kodak sold B&W papers.) If one ignores any HQ/Metol retention issues, I would imagine that the same modeling methods I've successfully used for silver in color systems would be equally valid for B&W.And, BTW, for good prints HQ and Metol retention are things to consider. As you re-use fix, these build up in the fix and there is NO test for them. So, be careful.