I'm skeptical of any tender that is rendered worthless by something as simple and common as an electrical outage.

And by "electrical outage" you mean more of a end of the world scenario? Because a normal electrical outage is not going to make your Monero worthless. It will have the exact same value before, during and after the outage. You need electricity to use Monero, but that can be also said about any credit or debit card.

I mean that I can promise to the merchant that I have monero I intend to pay them with but unless I can provide proof of this intention, by producing e.g. an IOU -- which will serve a purpose suspiciously similar to an official bank note -- they are worthless in the moments transaction.

What you are describing is also a limitation of Credit Cards and Apple Pay and Google Wallet. That does not make these things worthless either. Just wait a minute until power is restored.

Or pay in cash off course, but that only works if you remembered to go to an ATM first and that ATM had power.

Off course you could also write an IOU to the merchant about the amount of government IOUs that you will withdraw later.

I guess that means that your bank notes are worthless in the moments transaction if they are stored in a bank?

fine. bottle caps it is.