On Sun, 2 Mar 1997, Hans Aberg wrote:

>   Actually, what is needed, is a simple extension of the TeX \def command:
>     \def\name<parameter text>{definition text}[exception text]
> If a command somehow is unable to process the parameter text, then the
> original input text (before the parsing of the parameter text began) is
> left unaffected, and the execution passes to the exception text.
>   Then stuff like optional commands could be implemented most easily, and
> perhaps the stuff you are asking for, too.

This sounds for me like an OTP (Omega Translation Process) input filter.
With Omega you can do a lot of manipulation (notably character encoding
conversion and various more or less complicated character cluster->glyph
transformations e.g. for Hebrew or Arabic) of text before TeX sees the
text. You can avoid any active characters.