C.M.
someone once told me there are three things you just can't MAKE a baby do and they are: eat, sleep and poop. They never seem to do these things when we want them to. With that being said, I just don't know if there is anyway you can make her go before she lays down at night. What I would do since the rash sounds bad, is causing her pain and being in a dirty diaper for any period of time will not help the situation, is change her during the night. Put her to bed and then when you go to bed check her diaper, change her if necessary or if she's not dirty then, I would probably get up during the night to check it again. When you change her during the night keep lights as dim as possible, don't talk to her, leave her in her crib and before taking off the dirty diaper slide a clean one under her then take off her diaper, clean as quickly and gently as you can and then just fasten the clean one. I've been able to do it with them either not waking up or just stirring a little with no problem going back to sleep.
My two girls both slept through the night at 8 weeks so I know how you don't want to loose that, but I think she's probably been sleeping through the night long enough now to have developed a strong sleep pattern/habit that a gentle diaper change shouldn't disrupt her too much. Good luck - I hope the rash clears up soon.