After a lot of trial and error, I worked out how to change expressions in the 'take portrait' section, so for anyone else wondering why their avatar suddenly started looking like s/he's had a rough night on the turps:
1) Direction of gaze is changed by hovering mouse until the whole eyeball grid lights up, clicking and dragging
2) Upper eyelid/brow area can be raised and lowered
3) Lower eyelid can be raised and lowered - so, in various combinations of the two, your char can look sleepy, thoughtful, squint into the distance etc
4) The upper lip, when tugged, moves the mouth along a continuum from pouty to smiley. The lower lip doesn't seem to do anything (except move the head around, like all the non-adjustable areas in the headpose mode)
5) The chin, when tugged, opens and closes the mouth.
so
http://img28.imageshack.us/img28/3137/ciathree.jpg , with slightly narrowed eyes and the widest smile I could get looks happy (to me)
whereas
http://img837.imageshack.us/img837/6007/possiblecia.jpg with the same smile, lowered upper brow and no change under the eyes looks like she's fighting tears
Also, for those interested in the differences/ similarities between the ethnic groups, I made the same character twice, once as Deteis, once as Gallente
http://img98.imageshack.us/img98/586/charlieandcharlie.jpgSame tweaks to base model, same settings, same pose, same everything.
I'd be interested to see the results for other groups, too, if anyone's done them.