Dave,
Adeys suggestion is probably the best. Link to the Manfrotto page for details
http://www.manfrotto.com/Jahia/site/...id=107&idx=116 . It takes a moment to load.
Alternative without micro positioning is 357 UNIVERSAL SLIDING PLATE link
http://www.manfrotto.com/Jahia/site/...id=107&idx=115
The 357 is cheaper but not as effective as the 454 particularly at these magnifications.
I have had a play with reversing a 50mm on a 200mm and tripod mounted I set focus on the lens to roughly mid point. Slid my camera on 357 plate to reasonable focus then used the lens focussing ring to achieve fine focus.
Field of view of this combo in the viewfinder is 5 1/2mm on the horizontal so some serious magnification.
Don