Disqus provides comment threads for any URL, whether that URL is for a blog post or for a forum topic.
If you provide a way for users to create some kind of page (whether you call it a blog post or a forum topic), Disqus will then provide a comment thread for it.
Using Tumblr it is pretty easy to set up a blog that allows random people to submit new posts, which when approved show up as a new URL that could be considered a Forum Topic.
On Tumblr, random people can also ask questions of a blog, which when answered show up as new URL that could also be considered a Forum Topic.
The only downside is that all new Topics then require explicit moderator approval before they show up on the site, but given spam problems this isn't that big of a deal.
To get around even this one small downside, you could easily create a very small web app which merely allowed Disqus users to create new Topics, or pages, on the web app site.
Either way, the only real work that needs to be done is on the design side of things, making pages look more like the conversation is the primary focus rather than the original post topic. When you think about it, this is the only real difference between a forum and a blog as the information structure for both is the same.