I think you're asking for the impossible. To have a 76 key MIDI controller that is light and easy to carry is a non starter.

If I was in your shoes I would consider getting an Edirol or a Radium. Even though they don't have 76 keys, they have transpose buttons (problem solved!) plus lots of assignable knobs and sliders (which, funnily enough, the Roland ones do NOT have). They are also lightweight, portable and far more affordable than anything else out there at the moment.

Honest ask yourself... do you REALLY want to carry a 76 note keyboard around with you every time you have a band rehersal or a gig (along with all the tonnes of other stuff that will be part of your rig)?

Based on what you've said, I believe it may be in your interests to have a rethink about your entire set up.

-Equalizer