What are the requirements for a street side cluster box?

There is no requirement for how you use the box from an ADA standpoint. The only requirement for a pedestrian approach would be a clear floor space, accessible route, and reach ranges to the boxes. If it were a drive-up approach only, then none of these conditions would be required.

It is possible to have the USPS do door delivery due to disabilities. You would need to check with the local post office for procedures.