are there any plans to add the ability to upload offline posts
That may be a question to forward to either @PN_Jon or @PN_Jemma. Both are leading developers for the Juxtaposition Pretendo project.
By offline Miiverse posts, are you referring to making posts without an internet connection on a Wii U/3DS or are you referring to making posts on PC/mobile?
They are likely referring to the posts made during offline mode on a 3DS/Wii U.