For years I thought the small crack was nothing, and kept grinding it a little and doing a little filling. Each year when the boat was hauled, there would be a little weaping of liquid, sometimes with a greenish tint. I finally tore into it last winter and did about a 2'x2' square behind the keel and reglassed it all. As I ground, you could see the crack continue into the cored area on the starboard side.

I would do sections that I knew I could easily (with the help of another person) get back into place without much hassle. I think I probably went up to about 2'x2', and maybe even a bit longer if it was a skinnier piece. The trick is to not bite off more than you can easily get back on and hold in position while you set up the braces. I used a process similar to what you describe, but without the straps. I did a lot of work where the keel hindered the use of straps. But, I would use a backer, and had lots of long strips of wood that I would use to apply pressure and hold it in position. I would do the core in one step, and after it cured, I would fit the hull piece and sand down the core where needed to make everything fit just right.

I've tried replacing the core from the inside, and it's just too difficult to get where you need.