How to show declined meeting on Outlook calendar

Perhaps you should just accept as tentative.

What I was able to do here, is decline the invite in the calendar view, then find the invite (either in the deleted folder, or in my case it was still in my inbox for some reason), and tentatively accept but choose the option not to send to a response.

I think simple copy-paste is good enough.

• Before declining, click the appointment in calendar and with copy+paste you'll get a private appointment.
• Then you may decline the original leaving you with the copy in your calendar.
• If you prefer, you may change the copy to show as free in your calendar.

Some limitations are that you won't receive updates and you can't later on accept the copy but you'll have to find the original invitation from deleted items.

If you do "drag the declined request from the deleted items folder into the calendar" (tried in Outlook 2010) it will change "declined" to "accepted" and notify the mtg organizer.