Replacing tech reports: can we put good undergrad dissertations in arxiv?
Yes.

Any research writing (paper, report, monograph, dissertation, preprint, etc.) approaching publishable quality is welcome on ArXiv, whether its'written by tenured faculty, postdocs, graduate students, undergraduates, high school students, or hamsters.

(I am the ArXiv moderator for cs.DM and cs.CG.  I have not, as far as I know, accepted a paper from a hamster.)