I need to add a Trusted Location for Word which is a User GPO setting, BUT I only need this for about 400 users out of 3000 on our network.
The only identifier I have is that the users I want to target will some software installed on the pc.
Is there a way of setting up a User GPO setting based on the existence a given folder, so that I can target my setting rather than using a blanket approach?
No, you cannot. At least not only by GPO means. GPOs will always be applied if they match OU and security filtering. The latter only addresses more or less static sets of users or computers that are configured in the Active Directory.
So what you would need is a security group that contains only the 400 relevant PCs in order to let a GPO only address this set of PCs. However, this group is rather static in the AD and you need to fill it yourself, either manually or by some external program or script.
We for example use WMI to ask all PCs of the domain for their screen sizes and then make a security group that only contains the PCs that report a size of 19" or larger. This only works because the property we require is exposed by all PCs via the WMI that we can access remotely.
So you need some tool that iterates all PCs and can remotely determine whether the PC has the directory or not. One approach could be to add a login script via GPO to all machines that checks the property and connects to some central machine an reports its findings. You could either write some server process the login script could connect to or put the information by means of a file to some shared network drive where you can access it and create the list for the security group.
