Is it possible to specify which files are backed up to MASTER_SITE_BACKUP?

[Yuri]

I have the complex fetch stage which takes one set of distfiles, and 
produces another distfile, only that distfile is used in build.

There is the variable CKSUMFILES that controls which distfiles are 
included in distinfo (in my case all of them should be there). But how 
do I control which distfiles get to MASTER_SITE_BACKUP? Is there a 
variable that controls this, distinct from DISTFILES? In my case I only 
want the distfile that my fetch produces to be cached in 
MASTER_SITE_BACKUP. How can I do this?

Yuri