Hi there!
For me, this file doesn't need to be in a human readable format, but it must be created by non technical humans in a very easy way and read by machines, so with this premises, I would choose the easiest file format that a machine can read, but maintaining it's readability for technical humans. I think that is also important to make it lightweight to reduce network traffic overhead, so what about a zipped json file format? It is easy to unzip and to read by a machine, keeping it readable for humans that know what they are looking for.
For non technical people like business owners, they will need a web form to insert or update their data and store it on their website root, so this script should be placed on the same domain, included and configured by the website developer, and it should be error prone to future "server side language" changes or updates, and usable for non technical people like the business owners, so it must include as many input boxes as needed to define it's data format without mistakes.
The sad part is that even if many businesses implement it, crawlers will still need to look for other ways if it is not present, so it means more work for the crawling companies. Maybe you can provide ready to use libraries on different languages to make it easier to implement and to parse so more companies adopt it and increase the success chances.
Fernando.