Providing my own body text parts function

[Michael Grant via users]

In a body rule, SA uses the textual body of the message. 

From the docs: "The 'body' in this case is the textual parts of the message 
body; any non-text MIME parts are stripped, and the message decoded from 
Quoted-Printable or Base-64-encoded format if necessary.
The message Subject header is considered part of the body and becomes the first 
paragraph"

Is there a way I could provide my own function (override SA's internal 
function) to produce this textual representation myself?

Michael Grant