It's unfortunate that there aren't air filters that are easy to change. Toyota added them to later Prius models because of fans getting clogged. Why Lexus engineers haven't is a mystery to me. We have them under our front seats for the cooled seats.

 

If you look closely at the above picture then you see a filter in the middle. I think its for the Hybrid airflow. So apparently they did improve it in this model range. So, if that's the case I should be good to go and not have to worry as long as the filter gets changed at normal intervals.

 

The filter is for the air conditioner/purifier for the rear cabin. Per the text, the hybrid battery draws the filtered air from the cabin. So, there isn't a filter before the battery blower itself. The text also says that there's a service bulletin about cleaning the battery blower regularly. If one rides around with windows open on a regular basis, the blower sucks in dusty air. Even if that filter gets changed regularly. If I read post #7 wrong...
Hope you keep your windows up and all is well. I do agree that a clean interior will minimize an issue. My LS460 is 10 years old with 62k with a rear ac. The filter is original and still very clean as I examine it regularly if that's any help to you. My front cabin filter gets changed annually as the carbon wears out and doesn't freshen the air after a while. Even that doesn't get very dirty in dusty San Diego.