Custom Function for Virus Scanners Variable

[Kyle Harris]

I have created custom functions for several variables, however I am having
difficulties creating a custom function for the variable Virus Scanners.
When I create the function, MailScanner will not work (no errors and it
starts fine, just won't pick up mail in mqueue.in) even if I simply insert
one line telling it to return "none".  Has anyone successfully created a
function for the Virus Scanners variable or is this not possible for some
reason?

Thanks much in advance.