LinkAction (source) in the Sidebar Link plugin implements the Action interface (javadoc), which does not support link target frames. It is currently rendered using actions.jelly, which would allow the plugin to take over by providing a LinkAction/action.jelly file. However, to be consistent with how actions.jelly renders actions, LinkAction/action.jelly file would presumably have to use a l:task element, i.e. task.jelly, and that one doesn't support link target frames either.
So, implementing this would require either adding a target frame attribute to task.jelly in core, or duplicating the task.jelly DOM structure in the Sidebar Link plugin. The second option seems more risky because compatibility could break later if there are more changes like JENKINS-61973.
This came up in the comments on the plugin wiki page.. unfortunately I can't do this without some change in Hudson core. Actions provided to make these links do not accept other attributes like "target".
http://wiki.jenkins-ci.org/display/JENKINS/Sidebar-Link+Plugin?focusedCommentId=42470772#comment-42470772