One other consideration for rear derailleurs is the angle the parallelogram is designed to travel.
Modern derailleurs follow the cogs closely to maximize chain wrap (the number of teeth the chain engages on each cog) and will only handle a certain range of gears while maintaining this optimum. Granted, this is only crucial with newer designs such as SRAM or Shimano Shadow series that have a semi-fixed upper knuckle, but even with long cage derailleurs and sufficient chain length, the parallelogram angle remains the same and design limitations can be exceeded, causing chain jump, rubbing on the large cog, etc. Here's a diagram that shows the difference between "mountain" and "road" designs:
Whether certain combinations can be or should be run is up to how particular the individual is regarding wear, function, reliability and frequency of maintenance. Running components out of manufacturer's spec may work, but it's definitely a Class 'B' fit and will require a compromise somewhere.
Personally, were I finding the same limitations as the OP, I'd be looking at either a triple up front or an LX rear derailleur which can handle up to a 34t cog.