I can't cite any dictionary authority for this, but a 'cheerleading' sportscaster is commonly referred to as a homer. Google "sportscaster homer" for lots of examples.
Wikipedia does list this sense in its glossary of baseball, alongside the more familiar sense of "abbreviation for home run":
A home run.
Also, a derisive term for a dedicated, almost delusional, fan.
Especially used for a broadcaster, in any sport, whose team "can do no wrong".
Johnny Most of the Boston Celtics was a notorious "homer". In a somewhat more humorous example, Bert Wilson used to say, "I don't care who wins, as long as it's the Cubs!"
A common "homer" saying is, "My two favorite teams are (my team) and whoever's playing (my team's rival)."
Presumably derived from the sense of "rooting for the home team" (the team based in and representing the homer's city or location), with the common -er suffix of agency, indicating "a person who does or subscribes to".