The solution for me was to create a second invisible div at the end of the content of unknown length, this invisible div is the same size as my absolutely positioned div, this ensures that there is always a space at the end of my content for the absolutely positioned div.
This answer was previously provided here:
Prevent absolutely-positioned elements from overlapping with text
However I didn’t see (until now) how to apply it to a bottom right positioned div.
New structure is as follows:
<div id="outer" style="position: relative; width:450px; background-color:yellow;">
<p>Content of unknown length</p>
<div>Content of unknown height </div>
<div id="spacer" style="width: 200px; height: 25px; margin-right:0px;"></div>
<div style="position: absolute; right: 0; bottom: 0px; width: 200px; height: 20px; background-color:red;">bottom right</div>
</div>This seems to solve the issue.