Since segwit has already activated, such a fork would result in you losing any and all money that is in a segwit output, both nested and native. This is because such a fork without segwit would regard all of those outputs as anyone-can-spend outputs and the miners who initiated that fork would steal all coins in segwit outputs and send them to themselves.
To actually spend those, for native outputs, you would just have an empty scriptSig and it would work. For P2SH nested ones, you would just put the redeemscript in the scriptSig and it would work. There would be no place for signatures to go.
From what I understand in this part, you will not lose the coins and there's a workaround, but I don't really understand the process.
What you described needs to be done pre-split or can you do it post-split? (meaning that, if you are holding coins in native segwit addresses, you will not lose them in the fork that has no segwit) because if it requires you to do something pre-split in order to not lose the coins, this may be bad to convince people to use the native segwit format, specially long term hodlers that don't pay daily attention to what's going on and just sit and relax on their coins for years without checking news and price. Im sure there are a lot hodlers that still didn't even know Bitcoin Cash happened.