I haven't used those services, so I don't know their capabilities. However, a query on posts wouldn't work because a single post can contain multiple pocket operations due to editing. You really do have to query the blockchain directly.
But if there is a service that lets you see something about what is in a block before you download the whole thing, that could be very helpful. For example, if a block is outside the Genesis interval and it contains no comment ops, then it's not relevant to pocket. That sort of thing.
Also, you only need the first 30-40 characters of a pocketsend to check it it's valid. Don't know if that helps any.
I’ll investigate. I think there is a way to get the last version of a post. And maybe a way to get the block details by doing two calls, one to get a preview of the content and another one if we detect a comment op.