In the next GH version, there will be components that will update the definition as soon as a text file or an image is modified. Maybe these components will be useful in the meantime.
actually, there's one cool component made by Giulio Piacentino which can be found here, which basically adds a hacked up version of updating gh. It's really cool, I definitively recommend to at least look through it.
I've been looking at it today, it's really useful, i already did my own version in vb.net based on it (read 'based' as in copied). Besides the timer, a nice discovery was that its possible to store variables in a scripting component in between grasshopper updates.
It shows how to call the script component again for an update on the definition, so maybe there's no need for custom components after all.