There are all kinds of manhole covers and utility box covers installed all the time in drive aisles and other areas, including exit paths of travel, that would render the space unusable for temporary periods, perhaps shutting down the entire parking lot. Heck, leaves falling onto an accessible parking stall can make the surface technically noncompliant.
There is no prohibition that I know of in 2010 ADAS, the IBC or the CBC against utility covers (including grease interceptors.
ehilton has it right: the interceptor cover has to meet all the requirements for the surface: slip resistance, slope, maximum vertical change of 1/4", maximum gap width of 1/2", correct striping and markings (if located within those areas.
The other issue as it relates to ADA is: what happens when the grease interceptor is being pumped and someone wants to park in that ADA spot? Unlike when it is in the middle of the drive aisle and blocks all cars, this may block only the accessible stall while allowing others who have no disabilities to park in the rest of the lot. Now you have the potential for discrimination to exist during that time period. But this would be discrimination based on policy, not on design. For example of a solution, there could be a policy that the grease interceptor only gets emptied when the business is closed or the parking lot is entirely shut down.
So it is code-compliant at time of installation. It is certainly not ideal. And if I were a serial ADA litigant, I would go after the designer anyway, just because they might have deep-pockets insurance and I could make a tenuous connection to the service discrimination problem.