Aura is our brand for the abstraction layer and whatever firmware technology sits on top - it will run standalone but it's easier if you have OF or UEFI on top - this abstracts the hardware description so you don't need to know really which chip you're running on exactly (given that you can have gone through 4 or 5 chip revisions on something like the Beagleboard) or which revision of the board you're running on hardcoded or making a rough estimate in your code. U-Boot has the DTB for this but OpenFirmware preceded that specification by a good 15 years. We have a lot of experience here. Indeed, the HAL would have to target Aura.
Optimization is always an issue but you can boot any code build for previous ARM architectures - within reason - on newer ones. A kernel built for ARM5TE will run fine on ARM7A.. the idea is you can grab something you already got working et voila.. instant system. As it stands the similarity between the device tree Aura produces (based on OF) and the one bundled with the Beagleboard are similar enough that it should be able to support them both in the same ROM.
You would have to write some drivers, deal with quirks in the hardware that weren't abstracted (black/whitelisted USB devices etc.) but they are significantly reduced.