Most manufacturers of SBCs proudly supply their boards with a “Raspberry Pi compatible 40-pin header”.
But has anyone actually tried using a Raspberry Pi HAT on them?
I’m wondering, for example, if it would be possible to use a JustBoom DAC on an Orange Pi PC.
Any insights?
In theory yes, but the kernel would need drivers/modules added and built for each HAT. Thats a very long and painful road (at least for me lol) 
I’ve also just read that the connectors are not pin-to-pin compatible. They just share the same number of pins, and offer the same functionalities, but their placement on the board might differ. This kind of defeats the whole purpose (but I wonder why they don’t really make the connector pin-to-pin compatible… that would make things much easier).
The only way to use Orange Pi’s and other compatible boards for audio “works” at a near-hi-fi level would probably be to enable I2s on the boards and connect “generic” I2S DACs to the right pins.