For the optimized linetriple modes custom horizontal scale has already been added to the wiki Other feature request list.
However you can already get 1:1 pixel aspect:
linetriple generic 4:3 with h.samplerate 1110, h.synclen 30, H.active 1000, H.backporch 94.
linetriple 320×240 optimized mode with h.samplerate 341, h.synclen 15, H.active 320, H.backporch 17.
For linedouble you can set h.samplerate to 682 and h.active to 512 so you get 512×480. It depends on your display how that will be scaled. For example my monitor scales everything to 16:9.