Hey there Cory -
Eli here from the Shopify Guru team; when it comes to Shopify POS, what you're looking at is ultimately a mobile window into your Shopify storefront, designed for beautiful and efficient selling of product. As a result, there is no formal PoS API, in the same way that our core platform has an API. While this is partially due to function, as I just mentioned (the POS exists as a sales conduit more than a visual showpiece), it is also due to function.
Apps, for example, like the reviews app you mentioned, wouldn't really have a place in the PoS as customers aren't visiting the app to view reviews, for example. I spoke with our PoS team, and as a result of both these factors, there isn't a current place on the roadmap for an API that would allow apps to occupy a place in the PoS. I hope this answer hones in on what you were looking for; and as always, please feel free to let me know if there's anything I can do. :)
Cheers,