Service Level Agreements are now available on custom entities as of CRM Online 2016 Update 1 and CRM 2016 Service Pack 1 (on-premises). Even better, you can add custom KPIs to your SLA Items to monitor some additional parameters (datetime only, unfortunately, but I guess it’s due to the fact that time does not go back so your KPIs can be “crossed” only once).
To define custom KPI you only need to add a relationship between your entity and SLA KPI Instance. Easy said, easy done. There is a catch though. Your custom KPI will not appear in the list:
until you set the relationship as searchable.
Ugh. If this post saves you 30 minutes I spent hunting down the reason, I’m happy.