Hi Colin,
Sorry for the late reply (really late as a matter of fact). As far as I can tell reading through my code, the call is only stored to a WordPress transient if the cache_id attribute is set. So if you just don’t set that then I think it will make the call every time without caching the data. Obviously this will be slower but then you won’t have to update the cache_id constantly.
The other thing I think you can do is set cache_id="NULL" and that will have the same effect as leaving it out of the shortcode all together.
In the future I might add an explicit attribute for the shortcode that tells the user that nothing is being cached, but for now just leaving out the cache_id or setting it equal to cache_id="NULL" should do the trick.
Again really sorry for the late reply – for some reason I’m not subscribed to my own support forum but I’ll change that right now.