Interesting ! Not the typical application for a Slingbox, but I'll bite.
Other folks here may find this interesting if they would like to use their Slingbox for home security and monitoring...
You may be able to use Slingbox to get a low quality video feed from your live remote location back to the studio-- or wherever your webcast is being processed and served from.
It will not be a turn-key solution for your webcast. You're still going to need all the normal assets at the server as if you were doing a live production in the studio... can't use Slingbox for any of that stuff. You'll also need to pack the additional equipment required to get the Slingbox up and running in the field.
Networking is going to be the catch.
Basically, Slingbox could serve as a makeshift point-to-point link as long as you have reasonable internet bandwidth available on location. Ideally you would have an ethernet connection available, but WiFi is possible.
I wouldn't recommend trying it over the cell network unless you have the time to survey the sites in advance and your plan's bandwidth quota can handle something like this. Not to mention that you're going to have to get really creative in finding a way to grow an antenna out of the Sling's ethernet port. It's possible (I've done it) but it's a hemorrhoid. Also bear in mind that public events = tons of active cell phones, especially sports. It's one thing if a site checks out with an empty venue, it's something else if you park 100+ phones nearby and the tower starts to get busy.
So here's the deal-
Assuming you've got a way to deal with the networking, you need a working Slingbox on the camera side and either a computer with TV out or a Slingcatcher on the other side. Preferably a computer. Your camera should be able to feed the Slingbox something it likes as long as the cables fit in the holes. If you're ready to tackle a project like this, you already know what formats you're dealing with. On the other side, do the opposite. Take a composite (or component if your stuff can handle it) feed out to the video processor on the streaming server.
You're going to need someone at the server location to sling in to your remote to get things rolling. I suppose you could have the server already going and use some kind of remote desktop (VNC, RDC, logmein.com, etc) to do that, but you're asking for a headache.
I'd love to hear more. This sounds cool.
- Check out the Wikipedia page on Slingbox, down near the bottom there's a blurb about Broadcasting. Might find it interesting.
- Since this isn't a normal-scope support question, probably best to move the topic elsewhere. I recommend placeshiftingenthusiasts.com.