Well, I have indeed made fonts where the glyph was not what might have been expected in a proper Unicode font.
The examples that I can think of at the present time were for special purposes.
One is in the following post.
I suppose that if the Kombi font were going to be used with the version of Microsoft Paint on the Windows xp computer system here it would be easiest to have a version Kombi_8_bit where the alternate glyphs were mapped onto cells that Unicode uses for wavy brackets and the like for easy keyboard use and onto some of those accented letter code points that can be reached with an Alt code in Paint.
I do not know how Paint responds to Alt codes in later versions of Windows.
I have just tried an experiment and have been surprised and delighted at the result.
I used an Alt code, namely Alt 60001 to set up a character in WordPad using one of my fonts that has a glyph mapped at that codepoint, then copied onto the clipboard and then set up a text area in Paint for the same font and font size and pasted and the character pasted into Paint properly.
In case anybody wants to try repeating that experiment the font was the one at the following post.
William Overington
18 February 2013