Without some kind of a description/drawing of your proposed modification, it is impossible to even guess at how it will perform.  

I am not a SUP guy, but I race in an  association which includes SUPs.  For races with significant underwater obstructions, several of the SUP paddlers use pop-up fins.  I have seen both under hull and transom mounted varieties.  I suspect that this would be a better performing option than trying to design kind of long/shallow keel.  

Basically using the same dimension as the sheer line.  1/2 inch by 3/4 inche by 8 foot keel.  Id put a 45 degree edge on each side with a rounded top.   It's a 14 foot board so I'm hoping an 8 foot keep starting on the aft side would prevent the tail from kicking out as hard.   

Honestly, I expect that what you propose would perform very poorly.  The ultra low aspect (very long & shallow) keel would be high drag elative to the amount of lift that it would produce.  The area of the keel would also be significantly forward of the standard kee which would also decrease its effectiveness at helping the board track.  If you are not interested in the pop-up type fin, I'd suggest that you modify the standard fin shape so that it is better at shedding weeds.  Take a look at the shape of "weedless" rudders for surfskis.

   Search a forum discussion about this by Dan J. Bright.  I'm not sure how to link it, but if you search Kaholo 14 and look for him, you should find it.  He did what your're describing, and has great pics to document the process.  I contacted him by his email and he was very helpful.

Sean.