Easy to make the MC21 link plates to give more ride height, but as The Grantster says, calculating so as not to change spring rate (too much) is a little trickier, but certainly not impossible.
By changing the shock length you are also changing the spring rate to a small degree as the suspension will be working through a different part of it's stroke.
The way to do it is to make an accurate CAD drawing of the swingarm, linkage, frame mounts (top of shock and 'dog bone') and shock and then plot the shock length through various degrees of stroke. You'll see that for each degree of movement through the swingarm stroke the shock compresses more each time (hence, rising rate).
Once you have the points you plot the non-linear graph, then try to replicate that as close as possible for the new (increased) ride height.
Now, let's not get too finicky here. If you have an adjustable for length shock the actual change in rate over the 10mm or so length change that you're likely to use is so small that you wouldn't notice it. Rossi might!
But......I've already made a CAD drawing thanks to this thread (couldn't resist it), just need to put the points in and I'll know where we are.
I don't need different plates as I have F3 shocks, and Paul has an Ohlins, but it is an interesting subject and I quite like the idea of some fancy looking plates to bling my bike some more.
Question for the wrecking crew: Who's using ride height adjustment, and how much extra length are you using?