I use the flock blog editor to post to my wordpress blog (my self hosted one)
I use Custom Fields all the time - I have them for a lead story image, and for a lead video, if there is one, and a few other things, but every time i post a blog, i have to log into my wordpress and edit the post i just made to put in the data for these custom fields. it kinda makes it pointless to use the flock blog editor in the first place. so my suggestion is that you add support for addin a custom field to your blog post (and the associated data) it only needs to be two text input boxes, one that you can typr the name of the field, and the other to put the data in.
that would make flock truly the best app for bloging.

cheers.