PS, on the ISO 15919 wikipedia page, I notice a column labelled "7-bit ISO".  I have no idea where that comes from.  It bears no relation to what anyone actually does (HK, SLP, etc.)  Perhaps it's in the standard somewhere, but the ISO standard is paywalled and I don't care enough to cough up the 159 swiss francs.  

It's a long-standing scandal that ISO standards, which are ultimately paid for from our taxes, are not public.