I can feel twin bumps on either side of my lower spine which I believe are protrusions from the iliac crest -- approximately where the erector spinae appear to originate. I can generate pain in this region by sitting with my legs straight out in front of me and rotating one toe toward the other. Does this match the function of any specific muscle or tendon? I am unable to search for stretching exercises because I can't name the body part.
You would probably benefit from having a physical therapist's or chiropractic evaluation.
If you prefer to try to figure it out on your own search the site for piriformis for more information.
Since you've identified the bumps as the PSISs, and you reproduce your pain by rotating your foot (actually the hip into internal or medial rotation), look at the muscles that attach to the sacrum (which is part of the SI joint).
You will see that there are 6 muscles that laterally (externally) rotate the hip (toes out away from each other) with the piriformis attaching to the sacrum. If these are tight and you try to turn your toes inward, you pull on the lateral rotators that attach to the sacrum and can put a stress on the SI joint. The muscles work differently if the hip is flexed or extended.
You can stretch these muscles. You can massage these muscles. However, it is worth seeing a professional to have an evaluation and to set you up with appropriate stretches, strengthening exercises, and massage techniques with either a tennis ball/and or foam roller. A professional would be able check your alignments and isolate which muscles are weak and which are tight and decide if you need any stabilization. Hope that helps.
That is the "glute-med" as is colloquially referred to, more formally it is the gluteus medius.
To loosen this area try getting a "bumpy ball" and rolling it around there. Try not to use something like a lacrosse ball. This muscle is too small to safely withstand a lot of pressure from a single point.
As stated in a comment above, the issue may actually be with the SI joint and a professional should be consulted for further aid.