I was wondering how to post a subtree in org2blog for a while, and the answer is right there. I just needed to take the time to Read The Fine Manual:
Use M-x org2blog/wp-post-subtree.
Tags of the subtree will be used for tags.
Other properties can be set using a property drawer. The properties use the same names as the buffer variables.
POST_DATE sets the date to be used for the post. If it is not present, SCHEDULED, DEADLINE or any other active or inactive time-stamps are used.