Any civilization advanced enough to read microscopic encoded data would be advanced enough to do basic statistical analysis on ASCII encoded English/whatever and work out what it is. The harder part is figuring out how to understand English.
An example of this being done recently is MIT students decoding the ancient language Ugaritic
300M years is a very long time. Unless there is some continuity in the use of the stored information, in a way it's documented, translated and reinterpreted, there is no hope something alive 300M years from now, something that will not be remotely human, will be able to understand it any more than we understand the songs sponges sing each other.
Any civilization advanced enough to read microscopic encoded data would be advanced enough to do basic statistical analysis on ASCII encoded English/whatever and work out what it is. The harder part is figuring out how to understand English.
An example of this being done recently is MIT students decoding the ancient language Ugaritic
http://news.mit.edu/2010/ugaritic-barzilay-0630