4.21 - Textscreens
Text Screens must not have flickering effects, invisible or anything difficult to the eye. You are not allowed to place them on anyone's house or another players "Shop Area". Text Screens can not contain anything that is offensive or breaks TOU.
- Taken from the terms of use.
No disruptive, offensive or abusive behaviour: contributions must be constructive and polite, not mean-spirited or contributed with the intention of causing trouble.
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So TL:DR the question to ask is; why would you need to do this?
If you can come up with a decent cause to place a sign with those letters, i would say its allowed, but if its only to mislead potential raiders then it is not.