you have no programmatic access to cache from a WRT Widget.
Anyway, a possible way to override all cache-related problems would be to generate request URLs always different, in a way like this: just add, to every URL you call from your Widget, a custom parameter (e.g.: 'timestamp') with value equal to the current timestamp.
So, let's say you want to retrieve an image placed at:
you can actually call:
with the result that your image is actually retrieved and, being the used URL always different (since timestamp value will change), browser's cache will never be used.
Hope it helps,