This works exactly like Post Alerts work for Discussion Threads.
As you can see, the Alert for the new showcase item comment now contains a blurb that states that there may be more comments after this (just like the new blog entry comment alert, the new article comment alert and the new post alert below it).
There are no settings, options, permissions etc. The only thing to be aware of is that the PHRASE for the Alert has changed (so if you have translations, you will need to edit the phase and translate the additional new text that has been added).
Important Note: This is to be considered BETA and could be ROLLED BACK if there are any major bugs.