I current use a mix of XBMC and Plex but would like to be a Plex only user. One of the main addon's I use in XBMC is the smoothstreams.tv addon. It allows me to stream various sports (its a service I pay for) and uses RTMP/Flash/HTML5 to provide the streams depending where you are running it from (PC, XBMC or Phone/Tablet).
You can also stream inside VLC i beleive but I have not used this.
How would I go about developing a channel in Plex to achieve the same thing? Or can this not be done?
If they provide streams that are mobile-compatible, then it's likely do-able. Since it's a service that you pay for, it might be worth asking the service-provider to provide API access or even spearhead the development for the Plex channel themselves.
OK, so before I ask them if I did want to try it myself what would be the best way to approach it? Is there a template or guide that would give me a good start?
Yes they provide mobile-compatible streams via HTML5