If it were possible to alter an ASCII character without detection--that could only be "seen" by the intended receiver of a message--it would be ciphertext heaven.
Consider Bacon's Bilateral Cipher (example taken from here):
ITS A BEAUTIFUL DAY IN THE NEIGHBORHOOD
then the message (RHODES) is hidden as:
baaaa aabbb abbab aaabb aabaa baaab
ITSAB EAUTI FULDA YINTH ENEIG HBORH
Hidden according to this preshared table:
A aaaaa B aaaab C aaaba D aaabb E aabaa F aabab G aabba H aabbb I abaaa K abaab L ababa M ababb N abbaa O abbab P abbba Q abbbbR baaaa S baaab T baaba V baabb W babaa X babab Y babba Z babbb
When the proper spacing is put back in:
ITS A BEAUTIFUL DAY IN THE NEIGHBORHOOD.
So, is it possible to alter an ASCII character so that the receiver would know that it had been changed, but no one else?