I think the concern here is about what happens as an end result to the user. With reciprocity failure with long exposures on film we used to see shifts in colour balances, with Ektachromes and the shift to magenta and pale yellows with Fujichromes. There will be large artefacts too. I found almost predictable reciprocity failures when we push processed beyond 3 stops — especially in larger formats of 8x10in film sheets shot on Sinars.
So rather than what the technical process would be, it will be relevant to understand the end result. I found artefacts with Canon images shot for above 60 secs, more in the 5D Mark 2, much less in the 5D Mark 4's, probably because of the wider dynamic range. The medium format H6D 100C has a far wider dynamic range and will not see the reciprocity failure we experience. So basically digital offers us a far greater possibilities of success than film can do. Again, if the photons is sufficient. In other words, long exposures means sufficient photons, and therefore reciprocity failures of the colour casts I spoke of earlier will not manifest.
The larger digital backs as opposed to the DSLRs have amazing brightness ranges and are a delight to work with as the need for post editing with layered selection or even HDRs are not needed. A more realistic feel nearing film quality is experienced today.