Hard to say what the odds are for online registration. It is a popular race that is for sure. Only tips are to have your active account set up ahead of time and to be logged into active and waiting to register a few minutes ahead of when the online registration is slatted to open. Good luck, I’m bummed that I can’t down either.
IMLP is in a similar situation in terms of popularity. Lots predicted they would sell-out this year onsite, even the on the day of since they delayed the online registration by a few hours due to heavy onsite registration. Turns out the adminstration held spots for online registration. I suspect they will continue to hold spots for online registration for these events, just not a lot of them. The 800 spots they held at IMLP went in about 10 minutes. (I managed to register both myself and a friend in that time period.)
Doesn’t hurt to give it a shot, just make sure your account is set up, you have a fast connection, you have your cc and all the information you will need ready (don’t waste time filling out unnecessary items on the form), and follow the progress here, at the race website, and on active throughout the process leading up to online registration (in case online doesn’t start on schedule). This site is great for giving updates, but don’t always believe what you read here either, as posters are constantly stating “facts” that are based on extrapolation, speculation, and rumor.