Let me just say that Akoma Ntoso can be freely used by anybody. For example, your projet SayIt has used it without having to ask anybody. This to say that the list of organisations that use Akoma Ntoso is almost certainly an under-estimation of the actual use.
From my current understanding Akoma Ntoso is adopted even if at different level bythese people - not in any particular order, let's see...
The European Parliament, for modelling amendments, amendment lists, bills, proposals, consolidated version of those documents.
The Library Congress of Chile for managing debates and recently also bills and acts.
The Senate of Italy for publishing all parliamentary documents.
LexML Brazil uses a customization of Akoma Ntoso.
The Uruguay Parliament for modelling all the law-making process of the bills.
The Federal Chancellery of Switzerland for the publication of bills, acts, consolidated code in the Official Journals.
The European Commission is testing the adoption of Akoma Ntoso in order to better the interoperability with EU Parliament.
The Nicaragua Assembly for managing the whole life-cycle of law-making.
The Hong Kong Ministry of Justice.
Kenya Law Report is working to convert their own XML into an Akoma Ntoso compliant format.
Last but not least, there's Sayit. Oh, and also a customisation of Liquidfeedback by the Italian M5S.