The only suggestions I could find for Pushover were from late 2013, almost 3 years ago. Is it possible that Pushover support could be reconsidered? Perhaps as a VIP-only option, given that the stated objection was the cost of a higher API request limit.
Pushover is now available as a notification channel.
I'm looking into Pushover support. Just need to figure out the easiest way for users to connect their account.