You would rather not use the "Change status to" button?
You might find it much easier to set the default the value in the 'Change Status' popup list to be "Resolved" by merely setting up the manage configuration > workflow transitions: workflow 'default value' settings to be 'Resolved' wherever it makes sense.
ive done it this way so that the value is always the next logical step in the bugflow (and so i didnt have to go in and figure out how to add a new button, so as for that your on your own). i dont know why it Mantis isnt set up that way by defualt...
i set it up so that when the bug is 'resolved' the popup defaults to 'closed'. then when its 'closed' it defaults to 'feedback' (just like 'reopen' button does).
To add in dynamically shown buttons, my first approach would be to try to copy/paste the existing button stuff and custom tweak it to make it work.
However, im somewhat of a noob with all this stuff so im drawing blanks on how to approach this one. But hopefully someone else is more knowledgeable and can chime in.