"The main idea [of episciences.org] is to provide a technical platform of peer-reviewing; its purpose is to promote the emergence of epijournals, namely open access electronic journals taking their contents from preprints deposited in open archives such as arXiv or HAL, that have not been published elsewhere. The editorial boards of such epijournals organize peer reviewing and scientific discussion of selected or submitted preprints. Epijournals can thus be considered as “overlay journals” built above the open archives; they add value to these archives by attaching a scientific caution to the validated papers."
See also Mathematicians aim to take publishers out of publishing (Nature, 2013): "Mathematicians plan to launch a series of free open-access journals that will host their peer-reviewed articles on the preprint server arXiv. The project was publicly revealed yesterday in a blog post by Tim Gowers, a Fields Medal winner and mathematician at the University of Cambridge, UK."
Perhapsa physics epijournal (an arXiv-overlay journal with agile peer review) could be based on the Reviews section of Physics Overflow?