we almost always had great luck with the burt's bees diaper ointment, and it washes out of diapers very well. another one that did wonders was a wahmmade diaper salve available here -- http://hyenacart.com/BlessedBeeHerbs/index.php?c=0&p=.... it also washes out of cloth diapers just fine. if you're worried, fleece liners are a great idea and if they do get goopy -- no worries, just toss them because they are so cheap (or try boiling the buildup out if you want to). if the rash has bumps, it sounds like it might be yeasty, so putting on some lotrimin or similar (yes, the stuff for athelete's foot) under a thick layer of butt cream to protect it from further damage will probably help. also, warm baths with no soap and a few drops of tea tree oil in the water worked wonders when my kids had bad rashes, especially with yeast. and then ample bare-butt time like you mentioned you've already been doing.
as for the cause of the rash, make sure the diapers are stripped -- some detergents build up in certain fabrics and that can cause rashes. if too much detergent is being used, that can cause rashes as well because it might not all rinse out. also, some fabrics seem to hold onto bacteria more than others, and a tiny bit of bleach with extra hot rinses to get everything out might help out if it is something in the diapers themselves causing the rash. hth!