Interesting idea.... as a reply.   That has quite a bit of potential actually.  You could string multiple replies together if you needed to and fragment a larger data set across those replies.   THIS IS DANGEROUS talk though.   Something like this could be abused to attack the network thinking about it.

Because we are decentralized the witnesses still store the blocks.   What is to stop someone from making something that intentionally keeps creating blocks in the form of replies.   That's kind of scary, and hopefully something @dantheman has covered.    It wouldn't even have to be rapidly posted to be an issue... it'd just chew up a lot of space and the storage requirements for witnesses would balloon.   I don't know how the blockchain really works so there is hopefully a defense for this.

On the non-nefarious side of things, it could be a pretty cool little project and potentially be useful.

You could do some REALLY cool things such that your main post and the replies are linked together and a specialized website viewing program might be able to string some interesting data together.