As far as I can tell, there is no way to download directly to an SD card. I just tried it, using the Cydia tweak "Safari Download Manager." and, when offered the ability to change the file destination, the furthest back I could get in the folder hierarchy was /var/mobile. I was not able to "see" the SD card that I had mounted.
So, I don't think it's possible. I believe you still have to do the extra step of cutting/pasting. I could have missed a step, though, so I'm willing to be corrected, however...
Or, maybe the same kind of tweak for the Chrome app can do it. I don't have that tweak, so I couldn't try it out.
Marilyn