I have used three different methods.
1: Pull the Panniers off the bike and put clothes on top.
2: I generally have a chair with me. Clothes etc go of the chair.
3: I have used a second, light hammock hung beneath the one I sleep in to hold gear.
For changing, privacy, I use a large tarp and I carry two adjustable MSR poles. I can put one side of the tarp up, like an awning or down for privacy or to keep foul weather out.
Getting into your bag: I bring a mat with me to stand on to a: keep my feet clean and b: I will get into the bag while standing on the mat, sit down in the hammock, swing my feet up and then lay down. Then just zip the bag as you lay there.
"For those who fight for it life has a flavor the protected never know."
USMC 65-68 RVN 66-67