DeeDee249,
I've always assumed they were female longjohns, and as you suggest, any fly opening might have been commented on at autopsy.
Another perspective might be, did BR ever wear the female longjohns due to bed wetting, on an ad-hoc basis?
That is was he aware where they were located and familiar with their use?
.
I highly doubt it. JB was much smaller than BR. He would not have fit into her clothes.