The problem is not the transcoders (x2vga and VD-Z3) of not supporting 480i, because they do, it is your Monitor that does not support 480i. The reason is most monitors will not sync that low (480i = 15kHz). So, in able for 480i to be viewable on most monitors, it must be line doubled.
The N6 is the only device that i know of that has both transcoding and linedoubling features. So, it should work for both....