While it is true that you cannot create a SLA against the approval table, nothing prevents you from generating an SLA against the change_request itself (say, starting when the state changes to authorized, ending when the change state changes to scheduled, as this occurs when the change is approved). This definitely will handle requirements for single group approvals. You will likely need to be a bit more creative when it comes to multi-stage approvals, but it can be done.
You could easily create a custom field to store the current approval group, set it via the approval workflow, and update the SLA to retrigger on the field change.
