Depending on how many outputs you're using, output resolution and what type of interface, it could get complicated but I use the Blackmagic Design converters a lot to carry signal over long distance and the HD-SDI to HDMI (or Component) converters have two inputs (SDI and Alt SDI) and switch automatically if the main HD-SDI input fails. So depending on your needs, you could run two identical compositions side by side and if one computer crashes, the converters will switch to the output from your second media server. If you plan to use one media server and a backup image generator, you could distribute the backup signal using an SDI distribution converter with 8 outputs.
I've used that system many times but the other way around (into Resolume Arena via capture card) when I work with a satellite trucks. I'll have them pull two redundant HD-SDI signals in case there is a hardware failure somewhere between the truck and my media server. Thank god i've never had a failure but I've tested it and it works like a charm and switches over in just a few frames.
The other cheaper option if you use DVI and or HDMI would be to use a Kinivo 301BN or Kinivo 501BN since these little HDMI switches will switch automatically based on active input.
As far as back up source, here is a cool little device that one of my friends showed me: http://www.pocketvj.com/
I haven't tested it but it looks promising.