External Balance Control

Why not? You'll never know...

Why not just buy one (if they are truly transparent)

SMD Dact type 21 Stepped attenuator 
Buy two, one for left and one for right channel between pre and amp.

I wonder how hard it would be for Audiocubics to modify their external remote volume control to handle L/R balance instead of Up/Down.
Might be worth checking in with them:
http://www.audiocubics.com/http://www.audiocubics.com/specs.html

Thank you. I completely understand the “right” approach to address this need but I would like to keep my current preamp/amp. I know some folks use an in-line tone control device, e.g., Schiit Loki, etc., to tweak the sound and was wondering if there’s a similar gadget out there to just control the balance. I suppose it’s not that difficult or expensive to build a dual volume control box using premium grade attenuators such as Alps, etc., and that might be an option to pursue if there’s nothing out there for this specific purpose.

Kalali there are several passive preamps you could but in line in front of a stand alone amplifier. It is a bad idea to put a volume/balance potentiometer after the amplifier. The absolute best way to do this is with a preamp or integrated amp that has a remote control so that you can fine tune balance from your listening position. It actually changes from one recording to the next. Balancing a system so that both sides have exactly the same output is only a starting point.