Been down this road before (Hobie 17). The dolphin striker design is pretty much a perfect setup for galvanic corrosion...
The 20 setup is a little different than the 17 I believe, because on the 17, the bottom of the post is drilled and the striker rod passes through a hole in the post. On the 20, the bottom of the rod has a sleeve that has a hole in it for the rod and this sleeve slides over the end of the post to hold the two together. This is good because it means you may not need to completely disassemble the crossbar.
Part of the problem with the design (at least on the 17, I assume the 20 is similar, been a while since I've seen one up close) is that the thru holes in the crossbar are different sizes. The bottom hole is slightly larger than the diameter of the striker rod and the top hole is slightly larger then the O.D. of the compression sleeve. Once the bottom nut is installed on the striker post and the compression sleeve seizes to the post, you can't remove the striker rod. The only way to take the rod out without completely destroying it is by cutting the lower nut. On my 17, there were enough threads exposed at the bottom of the post that I could back off the lower nut maybe 1/8" just enough to cut it apart with a dremel tool and break it off the post. Then I was able to pull the entire post and frozen compression sleeve out the top of the crossbar.
Scroll down this topic, you'll see some pics of the striker post that was removed and how much corrosion you're up against.
viewtopic.php?f=15&t=53033&p=244101#p244101Whatever you can do to prevent having to disassemble the crossbar from the hulls is probably a good thing IMO, because there's a good chance the crossbar bolts are also frozen in the aluminum mounting blocks in the hulls which could be opening up a whole can of worms if you have to take that apart.
Also be sure to use a healthy coating of Tefgel on the threads and sleeve when you re-assemble to prevent this from happening again.
sm