Oops! I forgot about this post, but I wanted to answer in case it helps others in the future:
Thanks to rebe, I ended up bringing my own Charlie's Soap, which was a good idea. My MIL was squeamish about washing dirty diapers in the regular washer, so she actually bought a little mini-washer to wash the diapers in. (I didn't ask how much it cost--I hope not too much!) This seemed to work.
Our only issue was that it was the winter and there was no dryer and the diapers were not drying fast enough. I figured out I could dry them on a rack indoors overnight just beneath our room's heater unit. I did have to "supplement" occasionally with disposable diapers, though, when our CDs didn't dry on time.
Now we're getting ready for another trip. Our DD is partially EC'd and doesn't use as many diapers, but I'm going to bring some disposable Gdiapers along with the regular diapers, just in case.
Mama to a bilingual (Arabic/English) and cuddly 3 year old, and planning another peaceful homebirth in June.