I would first check if it qualifies for the late election.  Quite often, they don't qualify.

I don't have any first-hand experience, but I think I've read it is common to e-file with the election attached as a PDF.

Yes, enter that statement on the top.  In the event that ProSeries won't e-file it due to the 'red' error, you can try it again by disabling error-checking.