Believe it or not, the PSU will be absolutely fine mounted with the fan pulling in air from underneath without a filter or any kind of modification.
The rig will probably never use more than 50% of the PSUs total capacity, so the fan is likely to stay at its slowest speed for the entire time. So, this means that there's no need for a dust filter since the fan will only be spinning at maybe 600 to 800 RPM. Not only that, but the cables will be right next to the routing hole, and so that means only an inch or so of those cables will be showing as long as you pull them all through (which should be easy, even though it does require some physical labor).
I used to have the HX520W and the 9800 GTX+ in my system with exactly the way it is now since March 12th, 2008, and as near as I could tell, I could never get the fan to spin faster than its slowest speed. My ambient temperature is always 70-75Â°F. My system was just a piece of cake for it.
About 2 months ago, I swapped out the PSU for the HX650W to be prepared for the future (and also because a friend needed a really good PSU, so it was worth it). So the HX520W was in my system for over a year and a half, and it was mounted with the fan pulling the air in from underneath the case without any modification to that grill and without a dust filter. How much dust do you think I found? To my surprise, I couldn't see ANY dust. There was a little bit on the fan, but not enough to notice without careful study. But I guess I shouldn't be surprised considering that the fan was always spinning so slowly.
So there you have it: I am here to say that you basically have absolutely nothing to worry about.