[DNSB]

Dug out the Clara HD with 4.38.23038 and it works on there.

1. Go to home screen (not really needed but convenient)
2. Tap on the search icon
3. in where to search, Kobo Store, My Books, My Articles are tested to work
4. Type devmodeon in the search box (all lower case)
5. Tap on Go
6. The search will return no results found
7. Close the search window
8. Tap on More on the bottom line of the screen
9. Tap on Settings
10. Tap on Device Information
11. The last item in Settings will now be Developer Options (for the Clara HD, 3 pages of them)

I did not test this with SideLoadedMode enabled or other attempts to bypass registration. Since this is supported on 2 Kobos with 3.38.23038 and 1 Kobo with 4.37.23028, I feel safe in saying that it will work on any Kobo when you enter the correct term in the search box.