Ok, so I'm implementing your compulsory USB Power Management Guide for High Sierra.
After IOReg scrutiny I've found that in my case I'm in the same situation as your NUC:
"Note: You may find you have an EC in your DSDT: Device with "Name (_HID, EisaId ("PNP0C09"))", even if it is not active."
"Scope (_SB.PCI0.LPCB)
{
Device (H_EC)
{
Name (_HID, EisaId ("PNP0C09")) // _HID: Hardware ID
Name (_UID, One) // _UID: Unique ID
Method (_STA, 0, NotSerialized) // _STA: Status
}"
In my case:
"
Scope (_SB)
{
Device (PCI0)
{
Name (_HID, EisaId ("PNP0A08")) // _HID: Hardware ID
Name (_CID, EisaId ("PNP0A03")) // _CID: Compatible ID
Name (_ADR, Zero) // _ADR: Address
Method (^BN00, 0, NotSerialized)
..."
So I've added a "fake" one with the following SSDT-EC.dsl.
Created the file, compiled in aml, dropped the file into the Patched folder.
Now I can see in IOReg EC-->AppleBusPowerController
Having on this system a SMBIOS 18,1 I had to add a USBX SSDT and so I did copying the code from the iMac 17,1 you shared, then compiled in aml, dropped the file into the Patched folder.
Now I can see under EC-->AppleBusPowerController the properties I just injected.
So far so good I think.
Though I cannot see a "standalone" USBX@0" entry in IOReg as per the iMac 17,1 screenshot you shared before....If I understand I have instead a "fake" EC entry.