Thanks a lot Dasia, for taking the time to answer. 
Although it is possible to increment the page count 
"http:/username.tumbler.com/page/123" 
which leads to aforementioned "show more" stonewall, it doesn't work a level
deeper with "http:/username.tumblr.com/post/123456789"

As soon as I try to in- or decrement that url number in any way, it leads to a
404.
I guess there seems to be some algorithm or checksum involved.