Unless you just want to use a different firmware, why not just upgrade your dd-wrt build to one that has a working QoS? 17990 or greater on dd-wrt builds have a working QoS.

For one, i always watch the svn timeline for changes being made.

And for two, i'm a mod on the dd-wrt forums, so i try to stay informed. :)

Mainly because the builds past 14929 are more experimental builds, the 14929 builds seems to be considered most stable, but atleast for me, the builds past 14929 have more features, so i use them more often, plus i like to test newer builds - which should always be practiced with caution.

Also there is other things to consider here as well, like with the wnr3500L v1 - if i remember correctly here, can't run the big build past 16785, which to have a working QoS, you'd need to upgrade past that one. It can however run the std-usb-nas build, which should have most everything you'd need. Currently, i use 18024 - to my knowing, most everything works in it, but i don't use a WNR3500L to be able to tell you how 18024 would work on it.

Because of how netgear programs their boards, they use a small part of the flash rom to store the board config data. So in reality, you only get about 7.4 - maybe 7.5 flash available of the 8 that is there and the builds later than 16785 IIRC are like 7.6, so it won't fit anymore after that.

As i noted earlier, i don't use a WNR3500L v1, so i can't tell you if you could use your ext3 drive, i would think if the std-usb-nas build doesn't have ext3 support, that would mean no, you can't use it.