If it is the sealant, then yes you can get in there without removing the dash BUT, you will have limited space to work in when removing the rust using a drill and wire brush(if you don't intend on doing this- well then don't expect your repair to last longer then 6 months.)
If you have the rust issue- trust me, do it right first time. Or the problem WILL persist and get worse. The metal in that area isn't very thick so rust eats it away quite quickly. I helped a mates car and we did this, just patched it up with silicon as I was too scared to try remove the dash. When it leaked again 4 months later, we did it again, this time removed the dash, drilled the area out and cleaned as much rust as we could away using a wire brush, grinding stones etc. (Ideally we needed to take a grinder to it and cut the area out and re weld, but I'm not that advances, and then getting anti rust stuff on the back would have been a problem.) We actually ground out a piece slightly smalled the the palm of ones hand(rust was bad on the side we ouldn't see). Once we and finished grinding we got some rust buster and layered it EVERYWHERE, and tried to get as much as we could to run down the 'back' of the metal where we couldn't reach with the brush in case we had missed some. Once that had dried, we used a combination of sika flex silicone and fibreglass paste(fibral) to remake the piece we had removed and to cover the holes. We had to remove the windscreen tray as well to get in at the top to make sure it sealed. Was time consuming, but not exactly difficult. So far(almost a year later) he has no problems at all.
I'm willing to help you out via email etc if you want it man. You have my mail address, so if you got any questions, feel free to ask. I should of actually done a complete write up of how I did it on his car, but Ill need to get my laptop back from repairs first lol. Will also have to see if I can find the pics we took- I know I have some somewhere.