You can't use Contribute (using version CS3) to edit files with extension .txt.
If you can replace the extensions of the files you import in a flash swf, to
f.i. .php or .htm this solution maybe can work.
Your client will need a copy of Contribute to edit these files though.
I would suggest you download the Contribute trial and try if it works.

Consideration: As far as I know you can render files as html-text in flash.