No, but I can definitely relate to this from the other side of things. I've gotten into the habit of registering for lots of webinars that I will watch "if I am free" -- but that often doesn't pan out.
One thought here -- if you haven't already written something like this in your invites -- might be to try and make up the difference by encouraging more people to register for the on-demand/archive version:
"Can't make the webinar? Register anyways and watch on-demand later."
If you're integrated with your MAP, you could even make that a separate flow. (So they're not bothered with a lot of the pre-event reminders.)
Also, if you're integrated with your MAP, you might have more freedom to add a callout to a recent webinar(s) in your other marketing e-mails, or hit all of those people who didn't register with the webinars content in another form at a later date. (Whitepaper or summary.)
Same applies to your site. Where else can you drive people to register?
A final thought... some means of registering and then sharing the invite to another device, to watch on-demand on a mobile device later. I've made a personal effort to try and watch some webinars on my commute in on the train, but I think suggesting that -- and taking advantage of the fact that ON24 works well on mobile devices -- might also help drive on-demand or live views.