Yes, I think you are on the right track. I'm not a physicist, but it seems
that a liquid just isn't going to carry much charge. i think it is SURFACES
that can carry charge, and these surfaces have to have a distinctly
different polarity than what they are surrounded by. Anything works OK if
surrounded by vacuum (or air, even). The oil would have to be a VERY good
insulator, meaning very clean and dry. The metal balls ought to work, you'd
need a metal brush to break through the oil film to charge them.

Hmmm. Conductive rubber balls might work, if they're the right density and small enough to pass through the pump. Actually, I don't think they need to be very conductive because of the high voltage and low current. Slightly conductive would be enough.

It would be interesting to know how quickly the charge spreads to the outer surface of the oil column. Too fast to pump it up beforehand, I suspect.

Chris