Is there any way to "send mail as" from mail.app?

I believe you can change your email address Mail > Preferences > Accounts > Account Information.

to begin with, mail supports multiple accounts. so you can create an account that is set to never check its mail, and then compose mail from that account.

but furthermore, you can check and send your gmail through mail.app… so that could work too…

One caveat is if you are using gmail’s servers, you cannot use a different “from” address than the gmail account you’re logging in with.

So if I’m logging in as fluther at gmail dot com, I cannot send mail from jelly at gmail dot com (or jelly at fluther dot com), and if I try gmail will simply replace my adddress with the one I logged in as.

IIRC, in the “Email Address” field of the account information pane in prefs, you can enter multiple comma-separated email addresses. Then when sending you can choose any of them from the From pop-up.