16 years of BP ownership, bio-active is still the way.

[Malum Argenteum]

What effect would you expect all that excess nitrogen to have on the fauna and flora?

I don't imagine anyone knows the effects on a BP just from N in the substrate, but I doubt it is particularly harmful. There's definitely plenty of anecodotal evidence on the excess of soil N for plants (burning, especially if chemical N, and wild weak growth if from organic sources).

There's also anecdotal evidence that isopod cultures get tired after a handful of years, and I would expect that to happen in a system where there are lots of inputs and basically no outputs, though whether that's due to N I don't know -- probably general waste buildup. While isopods do eat their own and other animals' feces, these are not nutritional food sources and so the relative balance of feces to decent food might explain why old cultures get tired. They primarily eat fungus and cellulose (larger species eat wood and cork pretty readily), which is one reason leaf litter additions are another important part of nutrient cycling in these sorts of enclosures.

But the reason I picked N for calculations is that it is really simple to get good data on (we know how much is in a rat, and how much in plants -- and those were the only imports and exports). From those calculations, we can infer similar build up of other compounds that are more worrying, though we'd need to adjust our estimates based on how much of some analogous substance might be taken up by plants. Salts are a type of compound (mostly chloride and carbonate compounds) that build up in water-added soil systems that don't have exports of water (ever have a houseplant with no drainage hole in the pot?), which will eventually kill plants and likely microfauna. After pathogens, salts would be my next concern for a system with no drainage. Coco fiber and chips are known to regularly be high in salt levels, so such a substrate would start out at a disadvantage in this regard.

Not subject to analogy with N, but still a concern, would be the clogging of the void space in the substrate that would gradually reduce the ability of water to flow through the substrate, and reduce the ability of the microfauna to inhabit it. Long-term substrates (i.e. "ABG", which is the gold standard of organic substrates for this purpose) for waste-processing enclosures have a lot of void space -- they're basically uniformly sized charcoal chips and good orchid bark like Orchiata, with some peat and sphagnum to hold some water, and the best versions have hard tree fern fiber to serve as 'scaffolding' to hold the grains apart -- which enables water to flush sediments through and out. This clogging happens in houseplant soil, for another familiar illustration, but there we usually solve the issue with a repotting in fresh potting mix.