SLX chainring compatibility
I can't see how a flat inside ring with no pick-up points machined
  into it wouldn't be compatible, providing the bolt patterns match up
  (which they do, I think).


The bolt patterns are compatible. 

There are two reasons why the chainrings still may be counted as non-compatible:


Not all Shimano chainsets have identical fitting of the chainring, even if the bolt circle diameter is the same. Sometimes a chainring has to be slightly filed on the internal diameter. I tend to think that this clearly intentional incompatibility is not introduced without a reason and there is some other subtle difference. 

In practice: I used some "force-filed" chainrings and do not remember noticing poor shifting.
28T chainring will not be "hyperglide-compatible" with your larger chainring, whatever size it is. The latter is designed especially for upshifting from the 22T. Being shifted up from the (rightly positioned) 22T, the chain links hit the shifting pins and dents so that the chain engages properly. Being upshifted from another chainring, the chain links would not be correctly positioned (except if by luck).

This is the reason why, for example, the Shimano PDF which you linked lists two different 38T chainrings: "38T-AM" and "38T-AK". The former is "hyperglide-compatible" with 24T-AM, the latter with 26T-AK.

In practice: Some people would say that the difference is again not noticeable; here I disagree. IMO the front shifting degrades seriously.