Is it possible to change the alpha of a bar based on what modifier keys are being held down?

For instance, what if I just wanted a certain bar to show up while I held down shift or control... could I do that?

I use bongos now, and love the mod very much, but this sort of functionality would be enough for me to find a new bar mod.

It would be within the limits of the API, yes. I don't think such a mod currently (atleast to the best of my knowledge) exists however.

There are plenty of mods that do essentially that on mouseover etc., and converting to do it on a bound command probably wouldn't be very difficult. I haven't really kept up on the development of flexbar, but if it is in fact still being actively updated, thats likely a good place to start and or to appeal to the writer to add it.

Isn't Flexbar the one that's slightly more complicated than writing machine code? :x

Infinibar might be another to check out, also on wowace. Not sure if it supports that feature or not, but thought I would throw the name your way.

This can be done with Bartender (an ace addon). You can combine these features: (1) hiding the button 'grid' if no buttons are on the current bar, (2) leaving a bar empty, (3) adding a 'Button Paging' event to this bar, using a modifier key (as opposed to the alternative: stances), (4) filling in the destination bar with what you actually want on your bar.

The only disadvantage is that you effectively waste a bar (of which there are 10), which needs to remain empty for the bar to be "invisible" until you press the modifier.