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I believe part of the answer to your question is inherent in your post.
You included accented characters by way of example, which, since this
list does not permit HTML etc. means that you must have drawn them from
the standard ASCII character set. You either used an <alt> code on your
numeric keypad to produce them, or you imported them from a source which
was properly mapped to the correct ASCII character instead of being part
of a font definition.
My point is that the ASCII set, which is the basis of "plain text"
includes the most common (maybe all, but I don't know) accented
characters for Western European languages. Non-US keyboards simply map
their keys to different characters in the set. Thus the characters are
inherently available in plain text. It's harder for us English-speaking
types to get at them, but they are available on most PCs by holding down
the alt key and typing 3-numeral combinations on the numeric keypad,
such as "<alt> 138" and so forth.
Perhaps you knew this and I am misunderstanding the nature of the
question? I guess I only wanted to point out that a
work-around/convention is not technically necessary.