We currently use Ciscoworks LMS 2.6 with over 1000 devices in the database. I am pretty new to using this tool.
We may have to make a network wide configuration change to a standard access list that we use.
Is it possible to edit the configs of all devices, or a subset of devices, using Config Editor?
Yes, look in Resource Manager Essentials (RME)-->Config Management-->Netconfig
If you begin with creating a new job, you'll be given a selector to choose devices or groups of devices on the left, and predefined templates on the right. For ACL's I'd imagine you'll want to use AD-HOC which is just free text.
The online help is a good resource. Once you graduate from using the GUI you can create input files with the devices you want to modify, files with the commands you want run and run it via command line.