A couple of things:

Could you ask your daughter what it is about pants that upset her? Is it the material texture, the pressure at a certain point, how it clings or bunches?

For me personally, there were many clothes my parents couldn't force me to wear. My aunt had to force me to learn how to wear jeans at 12 years old. Eventually I learned to crave the consistant pressure of jeans up and down my legs, but as a child pants that were tight around my stomach area made me hysterical (they still do).

There are many materials that upset me due to pain and rashes. Polyester, Wool, and Angora give me hives. I can tell you the exact amount of materials in a piece of clothing based on how much pain and the type of pain I am in when I touch it.

Have you tried a weighted blanket or jacket yet? I definitely suspect some kind of tactile problems with your description of your daughter.

I use heavy quilts on my bed to weight it down at night. My always clothes cover specific areas of my body that NEED constant pressure. I wear sweaters sometimes in the summer time. It has to be unbearably stuffy and hot for me to not wear pants at all times. There are spots on my shoulders that MUST be covered no matter what I am wearing. This affects my bras, blouses, sweaters, and dresses. Strapless is impossible for me if I don't want to end up hysterical at some point.

Ask her what upsets her after she has had a tantrum and is now calmed down. Write down her answers and look for patterns, since she might not be able to articulate what is triggering her problems.

I used to complain that overly girly dresses had buttons that smelled funny and I refused to wear them. What was really going on where the overly girly dresses had a lot of lace and polyester. Both hurt like crazy and I couldn't express that at all. For some reason, smelly buttons seemed like a reasonable excuse to me as a very young child. So it can also be necessary to look at what she is complaining out, and not just what she says about it.

I thought adults were trying to kill me with all those Peanut Butter & Jelly sandwiches they gave me. I just told adults that I hated them, but I was really having an allergic reaction each time I ate them. No adult asked me why I hated them. I hope I would have been able to express that they made me puke, hurt my tummy, made me have to go to the bathroom with no warning, and it burned coming out.