I would definitely not recommend leaving them in until they "need" to be changed. Then really would run the risk of permanent damage to the head. I've changed the plugs in my 03 4.0L at 35k and 72k with no issues whatsoever. I have seen aluminum heads strip out twice now on trucks with around 100k miles and the original plugs (Both were F-150s, one 4.6L and one 5.4L)
My blow gun allows me to attach a standard brake line to it bent in whatever configuration I please to blow the crap from around the plug. Even if it wasn't, there no reason you can rig something up similiar to keep crap out of the cylinder. With the right combination of extensions, flex ratchets, and u-joints, you can change all 6 plugs while laying on top of the engine. It's not fun, but it's not impossible.