Hi Pippa
What a wonderful opportunity to look at SN from this perspective!
I can tell you what I think based on my experience as a yoga teacher, a pregnant woman and a perinatal yoga teacher (in that order)!
Firstly, practically speaking, you'd want to modify SN to make it possible/comfortable/safe for women in mid/late pregnancy. The strong initial back extension could overarch the lower back which is already under strain, so you might want to lift 'up' rather than 'back'. Uttanasana would not be possible without the legs apart to allow for the bump. Ditto for the step back/lunge, so you'd want the upper body more upright. Plank and dog would be okay for a woman who had done yoga before. However despite inversions such as dog being okay for women who have done them before, I'd ask why late in pregnancy you'd want to encourage the baby away from being head down in the pelvis, unless of course the baby was breech and you were hoping to turn it.
Secondly, are you trying to apply the sequence to the pregnant lady, or are you trying to find movements which are most helpful to a woman during pregnancy? What I'm saying is that there might be more beneficial movements such as various movements from table top position which give optimum foetal positioning for baby and takes the pressure off the woman's spine, or pelvic floor exercises to give both strength and flexibility to the pelvic floor for pregnancy, birthing and post natally. There are many yoga practices that are particuarly useful for a pregnant lady, which you'd have to apply based upon that individuals needs at that time.
So, taking into consideration; previous yoga practice, stage of pregnancy, needs as a pregnant woman and needs as an individual, you could decide whether to modify SN or do something else entirely.
Here's a link to a modified SN, which would be suitable if you had previous yoga experience: http://www.shaktiyoga.ca/YogaPoses/PrenatalSunSalutation.aspx
Birthlight teachers have a very gentle SN sequence suitable for all pregnant ladies which involves an initial stretch up, coming to all fours safely with the weight in both feet/hands, cat rolls, one foot up opening circles and coming back to standing safely with the weight in both feet. However there might be a lot of other things I could do with a pregnant lady which might be more beneficial.
I hope this helps!
Shanti
Liz.