I've also heard of using camping equipment waterproofing spray. Not sure it's worth it though. That stuff is usually fairly expensive and would be a temporary solution at best as I would expect that it washes off fairly easily (it wears off in one season or less on tents for example and they only get rained on at most, not put through multiple wash cycles in a machine rubbing against other clothes). Not to mention that the chemicals used may be iffy in terms of something to have against baby's skin 24/7 and rubbing off on other clothing or diapers in the wash.
I think the dryer drying is only for certain types of PUL but it's worth a shot, can't really hurt at this point... I'd try that and maybe some fabric softener. In theory if the cracks are thin enough the waxy build-up might help block them. But then it might also make things worse depending again on the type of PUL (can't recall the whole scientific explanation but build-up on some PUL types can cause them to leak).
Can you maybe reserve the cracked ones for daytime use when baby is getting changed very frequently? And then invest in one or two new (or gently used) covers for nighttime. Might work if the cracks aren't too too bad.