I picked up a San Marcos 51 cm for my girlfriend (1G light blue paint, 650b wheels). The 650b model has ample tire clearance (38 mm plus fenders) but the 700c models are a little tighter (35 mm with fenders). The fork is slightly lower-spec tubing (Tange Infinity), not that you'd notice without weighing it.
The San Marcos seems a bit more road-oriented than a Hilsen. It has a 6 degree top tube (two tubes in the top sizes) but otherwise its geometry falls between a Hilsen and a Roadeo. You could also see it as a budget Roadeo.
My girlfriend's San Marcos is built up as a light tourer, with drop bars and a front Campee lowrider rack. I'm about to change it from 8 speed STI to 9 or 10 speed bar ends (depending on which cassette I have lying around).
You're correct that the San Marcos doesn't show up on Riv's
frame page. I assume that a call to RIVHQ would answer the question of whether they'll continue to stock it. The Clem fits into the "entry-level Riv" price slot that the SM has occupied.