In Eyeshield 21, I saw Karin Koizuki as the Quarterback for the Teikou Alexanders. This is a start that a Manga shows a girl playing American football, which gives me very interest. However, I noticed that there were other anime and manga that shows female american football players. Here is a list of them so far:
Cat's Eye- episode 62 (Rui, Hitomi and Ai disguise themselves as football players to enter a football game for retriving a diamond)
Sailor Moon- episode 174, 184, & 194 (Seiya Kou (posing as a boy) becomes a football player
Magical Fairy Persia- episode 24 (Persia transforms into a football player to play in Riki's football game)
Hana O-En Step- Hitomi diguises herself as an injured football player to finish the football game
The B.B.B.-Tomomi imagines herself as a football player playing with her love, Shishimaru while in a game
Tennis Boy- a girl was playing american football before she joined a tennis team
Still, there must be other anime and manga about females playing american football. If you read this, then let me know what others are there.