Ok, so no simple solution?
Only the energy-mass-ratio of an orbit is certainly not enough since we can have very different orbits with the same energy. But yes, maybe combining it with angular momentum will yield something meaningful, so thank you for that idea!
Actually I want to land on Mercury after some swing-bys at Mercury and since this is very Delta-v-intensive, I want to be able to measure the "distance" between my orbit before/after the Mercury swing-by and the Mercury orbit in order to compare the quality of swing-by manouvers.