If NAS means a server with all the music on it, no I haven't tried it. With USB drives currently having 256 gigs and soon 512 gigs, I see no need with the number on USB inputs on the BDP-3.
Yes, I suppose flash drives are very convenient and available in 256/512 GB. As are SSDs or UBS powered hard drives in multiple TB range. Previously I had used those portable USB WD My Passport 2TB and 4TB variants.
I wanted to try out a NAS to see how the experience was on the BDP with it. There are some advantages to it such as not having to plug away drives when you need to transfer things. You can also remove acoustical noise from the room by moving the hard drives away from the room. Although, I suppose that doesn't matter if one is already using local SSD or flash drives.
The main reason that the NAS ended up being useful was for its SQ benefits over my USB storages (flash drives and portable hard drives). I do run a slightly unorthodox network setup, but it's relatively very cheap. Nonetheless, the hassle of getting the NAS, WD Red drives and spending time messing around with multiple network components has paid off sonically to my ears. Ultimately sound is what matters (to me) and that can trump convenience or simplicity. If you ever get a chance to try out a NAS share, give it a shot (and report back).
I'm happy with how cheap flash storage is finally starting to get. Cheers!