Pollen, unless blown by high wind, only tends to settle on things. Even a plastic garbage bag over your uncovered keyboards (not a cloth of any kind) will stop the pollen (and dust) settling down inside the keyboard, getting on key contacts, switches, sliders (especially vulnerable to contaminants), wheels etc..

Long term, you can get two plastic bags (I like the heavy duty garden leaf or big garbage can size), slip the keyboard in one, slip the other end in another (if the one is not big enough to close) and duct tape the two shut.

Pretty cheap long term storage, and proof against roaches and other creepy crawlys, too...