Assuming they are somehow translating the calls properly, dlss could work in proton, hardware regardless, but it would be much slower than current versions on rtx hardware with tensor cores, and probably slower than the earlier cuda core dlss also due to having to handle the calls properly. Dlss running on tensor cores is completed in just 1-2 ms due to tensor cores sacrificing color accuracy for speed. There's no hardware specific tricks, just low latency and a rtx card lock. Dlss can run on cuda cores/shader cores, and Nvidia's first release of 2.0 ran on cuda cores, but subsequent releases went to tensor cores, which meant instead of 6-10ms latency, you only had 2ms added dlss upscale latency at worst. In a handheld like the steam deck, I don't think dlss would be worth the added latency of translating the calls properly for the amd soc.
More likely, this is something for third party hardware developers who want to make steam decks with Nvidia discrete graphics, like a low wattage 3050/ti mobile gpu.