Subsets of Unicode and coding systems

[Ulrich Mueller]

Hi,
Sometimes I want to check if a file saved in UTF-8 encoding contains
only characters from a certain subset of the Unicode character
repertoire, like the MES-* [1] or WGL4 [2] subsets. (For example, for
things published on the WWW one might rather avoid exotic characters
for better compatibility.)

My idea was now to define a coding system e.g. for MES-2 and set the
buffer-file-coding-system accordingly, so that Emacs would check it
upon saving the file.

Now I have the following questions:
1. Is using coding systems for this purpose a reasonable approach
   at all, or is there a better way to achieve this?
2. Where can I find documentation how to define a coding system?
   The Elisp Reference Manual isn't really helpful, it only says:
   "How to define a coding system is an arcane matter, and is not
   documented here."

Ulrich

[1] <http://www.evertype.com/standards/iso10646/pdf/cwa13873.pdf>
[2] <http://en.wikipedia.org/wiki/Windows_Glyph_List_4>