The title says it all, I hoped to have a bit more ready but this is at least a bit over halfway to my stated goals.

The big news is the addition of a function type. This allows some interesting things like this:

def selector1(sdict):
 a = sdict['N']
 a = a % 2
 b = {'val':a}
 # return the index of the clip to select
 return b

def selector2(sdict):
 # for these functions the key 'N' holds the requested frame number
 # and FX corresponds to the index of the frames
 frame_properties = get_props(sdict['F0'])
 if frame_properties['SomeFrameProperty'] < 0.5:
 return {'val':0}
 else:
 return {'val':1}

# some source for clip0 and clip1 here
ret = core.std.SelectClip(clips=[clip0, clip1], src=[clip0], selector=selector1)

# ret will now return every other frame from clip0 and clip1 interleaved

# selector2 shows how frame properties can be factored into it