Is there a way to restrict/indicate a library is for the Mega only?

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2

Not exactly what you ask, but to make it very clear (rather than cryptic resources missing messages when you compile) you could just add

#ifndef __AVR_ATmega2560__
#error "please use an Arduino MEGA"
#endif

at the beginning of your library. That will stop the compiler if you are not compiling for a MEGA 2560, and the user will get an error message

exit status 1
#error "please use an Arduino MEGA"