Which explains how to show/hide the full taskbar and also gives the code and an example program that works nicely.
Maybe you could implement it into rainmeter and set it up with a hotkey.
Or if that's not plausible than my other idea was setting it up as 2 separate applications one a ( hide ) and one a ( show ), and then make buttons to use on the desktop to manually do it as you please. Or just as one application that will detect if it's hidden or not already and decide what to do with 1 click.
Or make it it's own rainmeter function to hide/show it
I know it's been one of the many things people have asked how to do and theres code out there to do it so shouldn't be too hard to implement it
If you do it like that and put Show.exe and Hide.exe in C:\Program Files\Rainmeter\Addons\ShowHide\ that would be best. Lets you use it from any skin and since the "addons" folder is a standard in Rainmeter 1.0+ it will make it easy to distribute the skin.