Microsoft have provided some interesting documentation on passing data to Cortana, in this example it is a flight schedule using Schema.org: https://msdn.microsoft.com/en-us/library/dn632191.aspx
Does anyone know if the same method is true for iOS and Android platforms?
I just reviewed the documentation, you've posted. Yes, you can integrate with Google Now in a similar way. You can find the supported Google Now schemas in the link below. You'll find microdata/json examples:
https://developers.google.com/schemas/now/cards
You can start testing Gmail Schemas with the below Apps Script Quickstart tutorial with your personal Gmail account. When the From: and To: fields match each other, actions will get displayed. However, if you try to send to another Gmail account, actions will not appear.
https://developers.google.com/schemas/tutorials/apps-script-tutorial
Note, if you simply copy and paste the examples in Apps Script, please use a current date in order to trigger a Now card. For example today's date:
"departureTime": "2015-06-22T20:15:00-08:00"
"arrivalTime": "2015-06-22T23:30:00-05:00"
If you plan to use Gmail Schemas for your users/customers, you'll have to register. You can find details on how to register at the link below:
https://developers.google.com/gmail/markup/registering-with-google
Note, you'll have to be within their guidelines and you will have to provide a sample email during the registration process.
Hope this points you to the right direction.