I have only implemented whatever i could find in Delphi 2 (the only Delphi i have available). I wrote a small demo "MDI doodle" application that uses all of the available APIs and tries to trigger various behaviors (e.g. showing/hiding a toolbar/status bar to update the client area, creating MDI children forms while existing ones are maximized, vetoing modifications via MDI childrens' CloseQuery, merging menus using GroupIndex, etc). The demo can be compiled in both Delphi 2 and Lazarus to compare the behavior of the two and is attached in the bug report above too.
The patch is written against SVN trunk at revision 62555. There might be some unnecessary tests in the code, but since i am not very familiar with it i decided to be a more cautious. Also since there is already an MDI implementation (AFAIK, didn't try it) for Qt, i decided to stick as much as i can on the widgetset side (the two main changes i made on the LCL side was to add an ArrangeIcons method in TCustomForm to mimic the Delphi one and add Merge/Unmerge methods in TMainMenu, again to mimic the Delphi methods and hopefully the functionality too :-P).
Feel free to remove any tests you think aren't necessary and make any other changes you think are worth it.