Whatever markup convention is finally settled on, I suggest avoiding in
it the use of characters that often get mangled when passing through
gateways or are national use characters in ISO 646. As near as I can
tell, those characters are
MIME says that they are
which is your list, plus quotation mark (") and commercial at (@).
The variable characters according to ISO 646 are all but the first two
of MIME's list.
Subtracting all of these from ASCII, we get:
% & ' ( ) * + , - . /
0 1 2 3 4 5 6 7 8 9 : ; < = > ?
A B C D E F G H I J K L M N O
P Q R S T U V W X Y Z _
a b c d e f g h i j k l m n o
p q r s t u v w x y z
So I guess these would be relatively safe to use in the enriched text