For reference, this little stunt is being performed on my 2006 1KRR. I'd guess this would work for all 2004-2007, but I have no clue about 2008 and later.
The first thing to understand is that we're dealing with a very simple latch here. Picture #1 below shows a good view of the latch. Normally, when you turn the key, the latch moves to the left (rider left) and lets go of the loop on the seat so you can remove it. Picture #2 shows a shot of the point on the loop where the latch grabs it.
We need to move this latch to the left even though the cable that connects it to the lock is broken. The problem is that the latch is buried inside the motorcycle and is not reachable. Or is it?
Look again at picture #2. If you look at the inset shot (looking almost straight down) you'll notice that for some reason Honda left a hole in the very thick steel plate that goes across here. Through that hole lies nothing but a little bit of foam and the vinyl cover on the outside.
So, a little bit of work with the digital calipers and I determined where on the outside that little opening on the inside would come out -- or more importantly where I'd need to go in if I wanted to come out through the little loop on the inside. The answers are shown in picture #3 -- 1.2 inches from the front edge, 5.2 inches from the left edge and 4.8 inches from the right edge. If those measurements from the left and right don't coincide, mark them both and then go halfway in between the two marks. In picture #3 you can see I've pulled the grab strap back and marked where I want my hole. Nice of Honda to cover the location for our hole with a strap so we can hide the little hole when we're done.
In picture #4 I've taken a straight, pointy, pick tool (an ice pick would do, but those are kinda large compared to this tool) and poked it through where my mark was. It went right through and out the other side.
If we were drilling a safe to spy the change key hole (safe techs will understand that reference) we'd call picture #5 the money shot. You can see that my pick came through right where I expected it to and is pointing right at the point on the loop where the latch grabs it. Of course with the seat installed you wouldn't see that.
Now the hard part. This is where a little lock picking experience is handy (or at least the ability to visualize mechanical things in your mind and translate those things to what you're feeling at the end of a tool). With the seat in place (of course it is or you wouldn't be doing this) poke the tool through. Because the latch is fairly narrow, chances are your pick is going to miss it and go all the way through until it strikes the end of the loop (as approximately shown in picture #5). From the outside, the bottom of the loop is at a depth of 2.1 inches. You should mark (with masking tape or something) a 2.1 inch mark on your tool. If your tool goes in this far, you're past the latch and bottomed on the loop. The thick, steel plate (with the oval hole in it) is at a depth of 1.1 inches. If your tool only goes in 1.1 inches and hits something solid, you missed the opening. The latch itself is at somewhere more than 1.1 inches and less than 2.1 inches deep. Feel with the tool to find the latch and try to push down from above and use the point of the tool to push the latch to the left. The spring is somewhat stiff, so you probably can't just mash straight down and expect it to move -- especially if you are using a fairly small, pointy, fragile tool like I was. I used the pointy tip of the tool to dig into the latch a little (by feel) and then pushed it to the left. I had it unlatched and seat off in about 10 seconds, but again, this is sort of similar to how you move pins around inside a pin and tumbler lock (blind) while picking it, so I may have better "feel" than some.
Anyway, that's it. Like I said, it's simple, but it may not be easy. When you're done, just put some black liquid electrical tape, or hot glue or black silicone or something over the hole to seal it (so water can't get into your seat foam). The grab strap will cover it and nobody will ever know that you punched that little hole.
Oh yeah - I said I'd tell you how to defend against this weakness. If you have an expensive Power Commander, alarm system, speedo healer, etc. under this seat (I have all 3 under there) and don't want somebody with an icepick (who has practiced a bit) to be able to pop your seat off in 10 seconds you can install countermeasures to prevent it. In picture #2 you see the very thick steel plate that surrounds the loop. If you unbolt this steel plate, you can then install a piece of sheet metal or something under the plate and covering the hole. Then screw the plate back in place and now you can't sneak a pick through there to remove the seat. Of course, if you do this, and your cable breaks, you're going to have to drill through whatever you installed to make a hole to get a pick through. That's not a big deal and you could still do it without destroying the seat. If you've got expensive goodies under your seat, this would be a countermeasure worth doing (instead of relying on "security through obscurity" which only protects you until some idiot like me puts just a little bit of thought into figuring out how to get around that security).