Thanks for posting this question Ken (and providing solutions).
It highlights the importance of not rushing anything at this time, as tempting as that may be. While you may see the reduction of salary to 80% as a quick way to eke out cashflow across a longer period of time, and keep employees in employment longer, a change process has to be implemented – and absolutely with full disclosure. The employee has the right to ask what you, as a business, have done to exhaust all other financial avenues before asking for 20% of their contracted salary. The ERA certainly would, retrospectively.
Also it would be good practice to ensure you have a fixed term applied to the time their salary is proposed to be reduced.
Another consideration is to propose that out the other side of this, the 20% is paid back on top of the return to contracted base salary (at agreed terms e.g spread across a period of time as long as it is fair and reasonable)
This is a good faith engagement. You’re both wanting to have a business and be employed through this and out the other side.