I recently spotted a pair of long disused 400w MV ballast cans and it turns out they were used for a interesting setup where the ballast cans were mounted on the side of the road and two overhead cables were used to feed a pair of 2nd generation Ovalite clamshells on a median mounted pole.

This vintage photo taken nearby shows this setup fully. You have a pair of clamshells mounted on the median and a pair of cables going back to the main power pole on the right to where the two pole mounted ballasts are. You can also see the more conventional setup for clamshells, with the ballast mounted at the base of the arm. Interesting they opted to mount the ballasts on the power pole instead of running a feed over to the median pole and putting the ballasts there instead.

Such a setup is possible as you can increase your ballast to lamp distance using thicker gauge cable but it seems like the setup would be a pain to troubleshoot as any issue with the ballast or PC (I suspect these were group switched back in the day) would require you to move over from the median to the side of the road.

It seems like back in the day people were pretty creative in remotely ballasting mercury luminaires, I've heard of mercury lamps designed to operate directly on a special series regulator so the series regulator serves effectively as the ballast for a string of luminaires. Line Material once sold such a system called the "Uni-Ballast".