This could be a fun contribution to do when I some day will regain my inspiration to hacking stuff. I've implemented this in the past using chump node module which is different than pushover-notificafions used by node-red-node-pushover. It unfortunately hasn't been updated in 3 years but it has support for receipts.
So this would not be very straightforward change to do unfortunately. Either the wrapped node library would need to be replaced, the feature contributed to pushover-notifications with the hope of getting it updated or by implementing the functionality manually.
There's another issue with node-red-node-pushover that kind of bugs me - the fact it doesn't use a config node so the API key needs to be manually added even after copying the node as it disappears...
