A maintenance release is valid to the hours/date of expiry, whatever comes first. End of story. Beyond that time/date the aircraft is without a valid maintenance release.
The ten percent often quoted and as equally often misinterpreted relates to various inspections called up in the MR. For example an oil change required at fifty hours into a 100 hour MR may be completed between 45 and 55 hours without penalty.
I guess an insurer would have an easy cop-out if the aircraft sustained damage without a valid MR. No doubt the owner would recover his damages from the pilot!
Dave,
Perhaps your company has put up a case relating to ground time. Saw a few aircraft where the operator recorded 90% of the tacho as flight time and such was entered on the MR. I suspect that practice has been stamped out in my neck of the woods and most operators have fitted an air switch to record flight hours for MR purposes - they still slug the student VDO!