I've got a Shimano Ultegra long cage rear derailleur on my tandem and I'm running a 11-32 tooth cassette with out any problems. Even though the specs say a 28 tooth cassette is the largest it will take, that's being very conservative, many folks are running larger cassettes.

I think it can handle a 34 tooth sprocket with some modifications. The chain will need to be lengthened and the "B" adjusting screw might have to be reversed giving you additional adjustment capabilities.

Hopefully others that have made this modification will chime in.