user/login service with custom hooks

I have a module that hooks into login (forms, using _form_user_login_alter, _form_user_login_block_alter and more...) to manage special cases (mainly external authentications): I've seen "services" perform just a plain "user_authenticate" call that has no hook available.

If login is performed via web form everything works (because custom hooks are invoked).
If login is performed via services doesn't work (because custom hooks are ignored as login doesn't use any form).

What would you suggest to solve this? I'm thinking of writing a custom resource that has username and password as input values and answers invoking drupal's own login form... any other way?

Thank you in advance.