How to cite a technical report for a double blind review process?
I hate to suggest it here, but in some ways your best option is to work to rewrite the paper within the page limits. It can be a difficult task to use fewer words without losing meaning, but it is part of the history of good writing to work at that. George Bernard Shaw famously said "I'm sorry this letter is so long, I didn't have time to make it shorter."

Mathematical proofs, if that is what it is, can often be shortened, especially if the techniques are standard and the audience is sophisticated. 

But the prose in the other ten pages may be more amenable to reworking. 

If you can do it the reviewers will be happier as will the rest of your audience.