Hi,
Looking at your example, I don't think you need to specify the contained in Windows Computer class. Instead you should directly reference the group.
Also, make sure that the sealed "Group of Servers located in another MP" contains objects of type Windows Computers class. Here is an simpler example I've tested in an unsealed MP and validated that it works:
A.Group - is basically the group I want with the logical disks. B.Group - is your group that contains Windows Computer object. I am assuming you are referring to Windows since you mentioned Windows Extended but can be also for UNIX/Linux devices. Since your group is sealed make sure you have the alias prefixing the group. In your case I believe it should be "***MPforNonProdSQLServersRegKey!***B.Group". I hope it's clear.