That wouldn't be my choice for material and application. Now, if you were going to lower the keel, clean the joint out, rebed it and bolt it back up, that's a good spot for 5200 because it can hold the keel on even without the bolts.
So this isn't the first time I've heard this. Where exactly did this rumor come from anyway? 5200 was what was keeping the ocean out of my leaky lapstrake 31ft boat that I lived on for 6 years and during 2 refits I used THREE CASES of 5200 on the hull. Both times I bedded the entire keel with it. When I say bedded I mean I used 5-6 tubes and put a 3/4inch thick layer spread over the top of the keel contact surface which was 8"wide and about 15' long.
Each time the keel came off instantly when the bolts were loosened and the keel weight was allowed to exert itself on the 5200. There wasn't even a 1/2 second of hesitation.
Even without the above experiment (which I performed twice) when you think about it logically it's just not possible. Even if it hadthe legendary adhesive properties that people claim, there is no STRENGTH to it. Ever held a cured bead in your hand? It has slightly less strength than a gummy worm. It can be easily snapped with finger pressure. Could gummy worms (suitably bonded of course) around each keel bolt hold up your keel? I think not.
There is many a legend about it's adhesive properties but I've also never had a hard time getting it off of any surface I stuck it on. I've never had it FALL OFF something it was stuck to, but that's about it.
I once removed several hundred bead-feet of it from my hull so I could recalk the seams. I would get a putty knife under it to separate it easily from the hull, and then I'd play a game of "how long of a bead can I peel off before it breaks." Sometimes it was only a few inches, sometimes I could pull a 10' bead of with gentle hand pressure. There really is nothing to the rumor that 5200 makes a semi-perminant bond. I still have 6 tubes left over and use it for damn near everything.
Now that I'm done wth my 5200 rant, I have enough experience with the product to answer the actual question that was asked.
Can you paint it: Yes. Several different paints have stuck well to it, though I would assume that hard paints (like pettit easypoxy) might be too hard and brittle and crack as the 5200 flexes I had great luck painting it with exactly that product as well as soft ablative bottom paints and also oil based house paint. Stood up well for years.