To expand on this issue, I'll include a few screen grabs taken just now.
One shows the very high domain authority from the example Tumblr site I referenced. (http://gothamlogos.tumblr.com/)
The other shows the snippet of code that links (outside of posts and image posts) to the sponsor site (http://underconsideration.com/), which -doesn't- have a "Tumblr redirect", and -doesn't- have a nofollow.
With these two screen shots, I became pretty excited at the thought of doing something very similar. However, heading into our Moz Prop account, I'm not seeing this Tumblr site passing any link juice to the sponsor site. (In fact, it's nowhere to be found at all in the Moz Pro reporting. See attached "Inbound Link Report" for underconsideration.com.)
Still holding out for anyone with this experience with this specific scenario before I mark this as "answered".
Greatly appreciated,
Zack
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