Rendering tofu instead of dropping missing font characters from output?

[sjamaan]

When I render a PDF which contains characters that don't exist in the font, these characters are completely stripped from the PDF.

Is there a way to render a notdef (aka tofu) character showing the code point, so that it is clear what is happening? This is especially a problem with user input, and causing bug reports to come in while it's not actually a bug.