2
I don't believe you can call a specific block using a URL.
What you CAN do, if you're not using all of your center blocks, is alter the theme template to move the code of one of the center blocks to the area want it to display...in your case, the logo/header area.
Then you would assign the XCGallery random thumbs block to display only in that block (which would now appear in the logo/header).
Of course, this wouldn't work if you're already using all of the block locations (center blocks - left, center blocks - center, center blocks - right) in your layout.
Anyway....kind of a pain, but it does get the job done.
JC
JC