Hi @Hoster ,
Unfortunately, it is my understanding that SPLA licensing with AzS HCI is prohibited per the Online Service Terms (OST). Servers running Azure Stack HCI OS must be dedicated to a customer's internal use and be registered to that customer's valid Azure subscription.
More information can be found in the OST:
https://www.microsoft.com/licensing/terms/welcome/welcomepage
https://www.microsoft.com/licensing/terms/en-US/productoffering/MicrosoftAzureServices/MCA
Hope this helps!
Trent