I don't think this is really a legitimate issue that needs to be enshrined in law - there are 100 senators so as long as there is a continuation of understanding around physical barriers to present yourself to a vote within the electronic realm it should work itself out. How different would a DDoS be from, for instance, one party hiring a mob of protestors to violently prevent law makers from submitting their votes in the statehouse - I'm not super familiar with the laws around this but I'd be surprised if there were no contingencies in place.

Additionally the Senate does recognize vote assignment as legitimate so it'd be reasonable to support pre-registration of voting or the allowance of a voting window.

Or a nation state kidnaps a senator, and vpn their vote back through the senators primary residence. They create a fake room that looks identical to the senators house. Any bad guy that's seen Mission Impossible one too many times would be inspired.

This could be the new way to filibuster! /s