I'm about sending PRs from Emacs i'm using every day.
Has anybody a patch that causes send-pr function declared in send-pr.el
to use functions from smtpmail.el?

IMHO it is not good practice to send PRs through local MTA
at least because SMTP servers often use authentification.
Those who use Emacs may prefer smtpmail-auth-credentials variable
eppeared in latest versions of Emacs.

I hope I'm not alone and this work is already done by somebody ;)

-- 
Best regards, Aleksey Cheusov.