assuming no color correction? i'd pull my SSS color from the color map.  i doubt it would be accurate, but it would be much more accurate than a flat or random color. that way i'd get red where the eye was red, and off-white where it was off-white.  if i were bagginsbill, i'd probably have different SSS workings for the different areas, because i'm betting the sclera has very different SSS values than the sort of fleshy stuff around it and connected to it.  i've seen maps for eyes where the back and edge area were obviously supposed to be very fleshy, and i'm guessing those should have a different treatment.

and just as  by the way, skin doesn't have red SSS either.  it has  a combination of yellow, waxy dermal SSS and red subdermal SSS.  since we don't have layers in Poser, the best way to deal with this is use orange.  i personally do something kind of crazy and complex to drive the hue with the original diffuse map with the SSS burned in.  it works pretty well for me, most of the time.