Nope, the suggestion has not been done before...
And it is in the correct forum if you want to suggest this as a Messenger Plus! feature (which might be possible).
But this can also be made as a plugin I think. The plugin would react on each conversation window when it is created (it's one of the API events if I'm not mistaken) and according to the settings of the user (he already may have disabled the showing of the user's DP) invoke the "hide DP" command for that specific window.
The best thing would be if it was added in MSN Messenger itself, I think. So you might wanna suggest it to Microsoft themselfs:
http://messenger.msn.com/help/contactus.aspx
I like this suggestion, no matter how it would be implemented. It's original, small , usefull, and doable without changing stuff too much, so
...
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