This is something I haven't tried, so you may have to experiment a bit.
The dataGrid is populated from a dataProvider. One possible dataProvider is the
HTTPService. The relevant parameters would be:
contentType="application/x-www-form-urlencoded|application/xml"resultFormat="object|array|xml|e4x|flashvars|text"
Since you can specify that the resultFormat is text,
you should be able to use that.