Here’s my use case: a client will be making numerous blog posts, some of which are portfolio pieces of his recent work. This sounds like a perfect use case for a Post Format, but no “portfolio” post format exists.
Post Formats are meant to be standardized, common formats, such as gallery, quote, status etc. Unfortunately “portfolio” isn’t in that list and doesn’t seem to fit with any of the sanctioned post formats.
Creating a custom post type for this seems like bringing a bomb to a bonfire and frankly doesn’t sit right with me since the portfolio blog post fits right in with the chronological main blog.
So should I consider extending post-formats (if that’s even possible)? Or is that strictly taboo? What’s another alternative?
You can’t create custom post-formats. What you could either do is have a post category called portfolio and then use the body/post class specific to that to control the display on the front end, as this is what post-formats are designed for, or go for a custom post type. Personally I go by the idea that, if it makes the posting screen less complicated for the client to work with for a specific post type, then a custom post type is the best option.
You can still include custom post types in your standard loop – http://justintadlock.com/archives/2010/02/02/showing-custom-post-types-on-your-home-blog-page so you wouldn’t lose that chronological order, it would just make it easier for the client to add portfolio posts, if they are to be so different to normal posts.