Can the main breaker in a sub panel exceed the ampacity of main breaker in the primary disconnect?

Yes. The main breaker in a sub-panel is merely acting as a disconnect. It's rating can be whatever you want since the breaker in the main panel is protecting the feeder.

Not that I'm asking, but I'm sure someone is :)

Here it goes: So does this mean I can protect my 100A non-service rated panel in my detached garage, using a 60A breaker at my service rated panel?

:)
Simply asking a question you already answered :)

Here it goes: So does this mean I can protect my 100A non-service rated panel in my detached garage, using a 60A breaker at my service rated panel? Yes, that is exactly what it means.