Based on a post at http://www.visualbasicscript.com/Getting-monitor-info-remotely-m96822.aspx, I seem to be getting fairly reasonable results for connected monitors using
unique values of (values "EDID" of keys "Device Parameters" of it as string) of keys whose (value "Class" of it as string = "Monitor" and exists key "Control" of it) of keys of key "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\DISPLAY" of native registry
The key being that items under HKLM\System\CurrentControlSet\Enum\DISPLAY<X><Y> are registered with Class=Monitor AND have a Control subkey. I also limit it to displays that report an EDID; this excludes my servers that are using KVM switches (which don’t report EDIDs) but is sufficient for my purpose. I have up to 8 monitors per workstation and so far this seems to be reporting back pretty reliably.
I’m trying to avoid WMI queries as I’m using this in a Fixlet Relevance to determine whether to install a monitor software management package, and WMI queries are expensive to use in Fixlet relevance.