I've noticed that at least in the first half of the first season in my ongoing TOS marathon that you do occasionally see a female crew member in thee pants variant. So at least the mini skirt is optional. Unlike TNG though we don't see any of the male crew in the mini skirt.
Yes, it was there in the background. I think what hurt the design was the collar was like that on the skirt uniform and too large for the tunic design. But it does indicate that with the right design a trouser variant for women is possible.
In the pilots the women's tunic was looser fitting than the men's and with a larger collar made it look more like a sweater. But the general idea is still there for a woman's variant of a tunic and trousers.