Alter an ASCII character without detection?

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:

…..R…..H…….O……..D……E…….S

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?