With CS5 you can use multiple pages for a single spread, and those page sizes can be mixed. So you'd set up 3 pages: back cover, spine, and front cover, in one spread. When you export this to PDF with "spreads" enabled, the crop marks appear for the spine as wel as the covers.

OR with any version you can do it with your method (faking it in as one page). To create the fold/crop marks, use the Line tool w/a .5 pt weight and manually draw these in where you want them. Drag them from somewhere inside the bleed allowance — close to the trim edge — out into the pasteboard a bit. Include a bleed measure (Export PDF > Marks and Bleeds) even if you don't have any artwork bleeding, otherwise the manual crop marks won't show in the PDF.

I do this alot to show folds (I use a dashed stroke for my line).

AM

Bob, excellent, excellent answer!  Of course the slug area is a smarter place to put your own hand-drawn marks (because some of the bleed allowance might end up in the final, trimmed piece — that's why you extend artwork into the bleed in the first place). 

And to use registration color for those, and .25 line weight.Thanks so much!

I forgot to mention that sometimes, it's quicker to drag out a frame that defines the spine area but extends into the pasteboard (slug area or bleed area) and then use the Make Cropmarks script on that frame. (It's one of the free scripts.) Delete the frame and you're done. Those marks are selectable so you can delete them, change their weight, etc. 

AM

Hahahaha yes you certainly do! That is impressive.

I would like to name my next dog Snippet, if it fits him/her well.

AM