The ROM login credits must be placed on the greeting screen (the first screen you see, excepting one-line "Do you have color?" messages), and must be viewable on a 24-line terminal with no scrollback.
The Diku login credits can be placed anywhere in the login sequence, which generally means either the greeting screen or the motd.
The Merc login credits are optional.
Note that the above refers specifically to the login credits, not the internal credits, which are a different issue.
The approach you have is probably the most common one, and is also the same as I used for my Diku derivatives.
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