So those reddish browns, particularly on the Imperial speederbike and Rebel landspeeder, anyone found good matches for those?

Can't speak to how well it might match the speeder, but a good start might be the Saddle Brown from Vallejo. It is a subtle red leather color. Could be mixed with basic red to increase the ratio of red:brown.

Vallejo beige brown washed with agrax earthshade worked great for me.