Well, let me know if you manage to downgrade to 9.2
I also have a brand new N3DS in 9.9. I also had the update nag cos I'm a dumb cow. But as they mention up there, if you use the TUBEHAX proxy (search for it) and you are fast enough in the startup you can bypass it. You will have to try several times, or many times, but it works.
I went ahead and immediately installed menuhax (so keep it also in your SD, because that's the first thing you have to do once browserhax runs)
I can run the homebrew channel in my N3DS 9.9. I did this last Friday.
But downgrading ISN'T that easy.
First of all, it fails loads in N3DS. It seems to me that people with software greater than 10 and lower than 10.3 has been successful using the latest version of sysUpdate. Everytime I ran sysUpdate (I've tried several versions) it fails at a random point.
Beyond this is where you need a master degree in gbatemp to undestand what is going on: safeSysUpdate seems to work better for people in 9.3-9.9. Some reports issues. Then you mix in miniPasta for God knows what reason, then you see people first UPgrading to 10.3 and then DOWNgrading to 9.2 to bypass the issue with the lastest sysUpdate, and lastly you get scared with posts of people having softbricks and having to update to 10.5 and saying goodbye to the scene for some time....
I've got a gateway and I think I'll wait some weeks to see if they release something. If nothing moves after a couple of months, I guess I'll risk downgrading...
Anyways, let me know how it goes!