There are already ambiguous post rating buttons that could serve this purpose. Bucket of Rainbows and You Get a Cookie can both be thrown randomly at posts without necessarily inferring any particular response to the message.
Alternatively, a like rating sounds perfectly reasonable to acknowledge someone saying when they're going to post. GMs like having communication and people posting in their roleplays, no?
A thank rating for the notice of absence could work too, as in "thanks for letting me know."
Adding some kind of generic notification would be nearly worthless clutter given the fact that post ratings already exist and we already have more of those than is really needed.
