Ummmm..... use the centerboard to help get onto a plane in marginal conditions.....very doubtful that this will work.
The CB causes alot of drag, so as soon as you think you have enough wind to pump onto a plane, get that centerboard up out of the water and "go for it".
Now, then, if you think that "railing" the Rio up onto lee rail with the centerboard down is "planing" , there is that aspect, but it's not truly planing using the fin only, and the board doesn't really unwet and plane, it simply gets alot of speed because you are developing alot of lift with the CB by "railing" the board to leeward.
In my opinion, unless you are on a true longboard, the center board needs to be either all the way up or all the way down. There are situations in longboard racing where a partially deployed CB gives a more desireable amount of lift when going fast, but a partially deployed CB makes the board very hard to turn and maneuver.
The Rio should plane nicely on only the rear fin, and if you want to get the most early planing, then I'd suggest a fin in the 58 cm range.
Lots more lift, and it will get the board planing a couple of knots earlier than the stock 41 cm shallow water "cleaver" fin.
I sail the Rio M and it plans quite early and it's resonably fast for an oversized shortboard with a mounted centerboard.
If you use the larger 58-60 cm fin, you will be able to get "up on the fin" and go upwind much better with the lee rail lowered slightly and pushing very hard across the top of the fin.
Hope this helps,