I don't think there's away to do it in the GUI. If you are using OSX try this AppleScript. You would have to be careful with reflows after running this because the changed paragraphs would remain fully justified even if they shouldn't be
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--adjust this to desired percentage. Any last line that's wider than 90% of the column width gets justified
set zone to 0.90
tell application "Adobe InDesign CS6"
set paras to object reference of every paragraph of every story of active document
--check the length of each last line
repeat with x in paras
--get the full column width of the paragraph
set i to horizontal offset of first character of line -2 of x
set j to horizontal offset of last character of line -2 of x
set colw to j - i
--get the last line width
set a to horizontal offset of first character of line -1 of x
set b to horizontal offset of last character of line -1 of x
set lastw to b - a
--if the last line percentage is greater than zone number justify
if lastw / colw is greater than zone then
set justification of x to fully justified
end if
end repeat
end tell