First off, Thank you for all your work with the Asus firmware, i am really looking forward to your next release :-)
I don't think its necessary to open the 6882-6899 port range. My experience is that its only necessary to open the port specified in the MLDonkey config, which i guess is 6882 as default.
But if it is possible, it would be a good idea to change the default port to something else, because many ISP limits the speed on the standard bittorrent ports. I use port 3400, but some random port above the standard service ports like 54286 or something, would be great. And as said earlier, if you specify port 54286 as bittorrent port, it should only be necessary to open up that port.
Uh and one thing more. What version of MLDonkey are you intergrating? The 2.8.5 version has buggy bittorrent code, which results in slow speed, and the 2.9.2 is just acting weird when adding torrents. In my experience, the 2.9.1 version is the one working the best, reaching speeds of several hundred kb/s.