Preventing the thumbturn on deadbolts from turning is really the only way to stop lock bumping or picking - but only when the locked room is occupied. Chair-under-doorknob is how I've always done it (especially with interconnecting hotel room doors; I never trust those weird one-sided knobs) - though I admit no red team testing has ever been done to quantify the efficacy of this method.
These guys have devices that do essentially the same thing and they all appear marketed heavily towards 'travelers' or others who find themselves sleeping or bathing in unfamiliar places:
Super Grip Lock - over the thumbturn strap
Add-A-Lock - linchpin style
Lock Jaw - slide-over form factor like the LockLocker
Of course, they all suffer the drawbacks you mention above by preventing the normal deadbolt locking mechanism from working.