Therefore, I've just written a small Java program which can merge two keystores together. The source is available here. I've also just released a new key package for my keys using it here; I hope others will do the same!
For non-developers: there's no real need to upgrade to my new keystore release as the keys are the same and provides no new functionality to my apps.
For developers: It assumes the keystore and keys all have a password of "password". The command syntax is:
mergekeystores <dest keystore> <merge keystore>
This will merge keys in <merge keystore> with those already in <dest keystore>, or create a new <dest keystore> if one does not already exist. All you need to do is create a keystore of your own, with your own unique alias string, and create an update package for them.