As always, anyone with osteoporosis to any degree should check with their health care provider before beginning any yoga practice. It would also be a very good idea to know their Dexa score so they can tell their yoga teacher as well (a Dexa scan reveals the degree of degeneration in the lumbar spine, which is a good indicator of overall bone density.)
An excellent resource to understand yoga (in general) and osteoporosis, or even milder osteopenia, is Dr Loren Fishman's upcoming book Yoga and Osteoporosis. You don't have to wait for the book: you can check his web site for stuff now:
http://www.sciatica.org.
To avoid any possibility of a "wedge fracture" of the vertebrae, all forward bends should be strictly avoided if you have osteopenia or osteoporosis. this includes Yin Yoga forward bends. A wedge fracture is where the vertebra gets thinner in the front of the spine, until eventually it fails and factures. Since this is the front of the spine, forward bends put extra pressure onto the degenerating bone.
Now, it is possible to strengthen the bones, and Yin Yoga is an excellent way to do just that. Dr Fishman has many yang ideas as well. Anything that creates compression on the bones helps. Yang yoga will use the muscles to create that compression. Yin yoga will use passive postures to do the same thing, and for longer periods. Just don't do any forward bends, and go very gently!
Re hypertension: usually we are concerned if the head is below the heart, which occurs in inversions, including mild ones like downdog. I am not aware of any studies that show forward bending increases blood pressure, if the head is not below the heart (ie: Caterpillar shouldn't increase blood pressure to the head, Dangling will.)
Cheers
Bernie