No, it's not a word currently in use.
Also, I can't find an existing name for the ends of the minor axis of an orbit (or even an ellipse). As you noted there doesn't seem to be much use for the point, which would be why it's not named.
As TildalWave pointed out in a comment the general word would be mediapsis (-helion being the -apsis suffix for orbits around the sun), and that doesn't actually make sense when considering the modern use of -apsis (as in "apoapsis" and "periapsis"). To quote the wikipedia page for "Apsis":
An apsis (Greek ἁψίς, gen. ἁψίδος), plural apsides (/ˈæpsɨdiːz/; Greek: ἁψίδες), is a point of least or greatest distance of a body in an elliptic orbit about a larger body.
On the other hand, in a comment Mark pointed out that the etymology of apsis indicates that other usage besides peri- and apo- would be valid:
The term apsis is derived from the Latin word apsis, meaning "arch" or "vault," which can be traced back to the Greek word hapsis, meaning "loop" or "arch,"...
The lack of an existing name means if you want a name you'll have to make it up and "medihelion" at least sounds like part of an orbit (and arguably works as a use of the -apsis suffix). I'd recommend providing (or at least implying) a definition to mitigate the risk of people trying to look it up and being frustrated when they can't find it.