Hi, I agree.
I am nearly 8 months pregnant with twins and have taught Yin (and Yang) for nearly 4 years.
I still love practicing Yin poses, but because of the loosening in the groin connective tissue, 1-3 minutes is more benefical. Some of the poses such as half butterfly I will hold for 5 occasionally, but to be on the safe side putting a blanket under the folded leg can prevent over-stretching.
I would not encourage a student to hold for more than 1-3 minutes. In fact, in swan 1-2 minutes seems to be quite enough. I do this pose in almost every prenatal lesson, as the release on the sacro-iliac joint is heavenly for most... But due to the relaxed tissues of the pelvis, some people may find it very uncomfortable. I would say always be cautious and ask the students how they are feeling regularly, as SPD is quite common and very painful.
There is a lot of benefit in practicing prenatal Yoga with a Yin minset though, noticing tension and exhaling it out, working with the sensations of the body rather than habitually moving. My students tell me that this breathing technique helped them in labour immensely and reduced their need for pain relieving medication. This can be done in heavily supported poses though. I would encourage students to be comfortable using as many blankets, blocks bolsters as they like so that they can relax into poses without overdoing stretches.
Another thing worth mentioning is that from before I was even showing, in the first trimester, I struggled with my favourite Yin poses; seal and saddle. I was always more of a back bender.. but now anything other than the cat cow, standing cobra or an occasional reclining half saddle on a bolster (for 1-3 minutes) doesn't feel good to me. I have had no back pain at all during pregnancy, which is strange for me as usually if I skipped the back bends I felt it... Maybe it's the twin pregnancy as there's not a lot of room left in my tummy to arch backwards now, but I haven't had a singleton pregnancy to compare my experience to.
As I move more towards the end of my pregnancy Yoga Nidra and gentle stretching are the main part of my practice.
Hope that this helps some people... Feel free to ask any questions