Couple options, first one already mentioned:
<1> Bogen SuperClamp + Bogen Extension (35-92")I use a SuperClamp + custom extension pretty regularly, but the Bogen extension is cheaper and should work just fine. I clamp to a cylindrical horizontal bar in the balcony, which allows me to rotate the clamp around the bar to adjust the extension's angle / depth relative to the mount point.
Pic (thanks, Ed!) without the bar to which I'm clamping.
This setup works well if you're able to clamp to a horizontal, cylindrical bar. If you need to clamp to a vertical bar, or a horizontal bar that's not cylindrical (like a square-ish bar) and therefore won't allow you to rotate the clamp, you won't have the flexibility you need to adjust the angle / depth of the extension. In that case, try option <2>...
<2> SuperClamp + Posi-Lok + ExtensionThis provides greater flexibility. Clamp to the rail so the standard stud is horizontal. Attach the Posi-Lok to the standard stud, using a jam nut to align the Posi-Lok so it rotates into the plane you want. (I like
Latch Lake jam nuts). Then attach the extension to the Posi-Lok. The Posi-Lok allows you to control the angle / depth of the extension as it runs down from the mount point.
Personally, I wouldn't use my C-Type clamp to secure anything other than the mics:
I just don't feel it's robust enough to hold the added weight of the extension + the mics and the associated torque with the mics extended.
Note 1: In both cases above, you may need various adapters to attach the extension or Posi-Lok to the SuperClamp stud.
NOTE 2: In ALL cases of clamping to a balcony railing, or any position that hangs your mics above the crowd, you should use a safety cable. Clamp falling or getting knocked off the raili ng and the mics or your clamp clobbering someone in the head is a good way to convince the venue to shut down taping altogether. In addition to safety, it demonstrates professionalism and will help set the venue's concerns at ease.