EA SPORTS has just notified us that we can both confirm and freely talk about anything that has to do with Custom Playbooks. Please feel free to post any questions, thoughts, and concerns that you may have within the comments.
~ Total plays - The max number is 377, but it varies between 370-377 based on the teams base playbook.
~ Playbooks - 15 on offense 15 on defense
~ 40 formations total
~ Can be used in Online Dynasty and ranked/unranked online games
~ Goalline and Special teams are in each playbook
~ The max number of formations will depend on the playbook.
~ Cannot edit Goal Line, Special Teams or Hail Mary. The number of plays in defensive playbooks is also dependent upon the base playbook selected so it falls into the 377-370 range as well.
~...Also, I found this to be pretty handy, but your Custom Playbooks will show up after the Z's and before the A's in the order at the team selection screen. I think this is handier than alphabetical order because it will group all of your custom playbooks back-to-back so you can easily go from one to the other (when you have defaulted one of your custom playbooks as your default playbook).
~...unfortunately there won't be any easy way to share custom playbooks. I'm not sure if you will be able to transfer save files or something between consoles or not but there is no dedicated online sharing like rosters has.