Can yum operate relative to a user-specified state?

Hi everyone,

I have an isolated network of computers (i.e. behind a 'firewall' made 
of air so no access to the net) but would like to be able to use yum on 
the network.  Simple enough - I have set up a yum server on this 
network.  My issue is that I would like to avoid maintaining a full yum 
mirror.  A full mirror would waste resources (time, disk space, net 
usage) because I would be downloading and transferring gigabytes of rpms 
that aren't actually required.

The solution I have in mind is to use yum on a different computer that 
is attached to the net to download the necessary packages and then 
transfer them.  However, this computer won't have the same set up: it is 
i386, not x86_64; Fedora 10, not Fedora 11; has different packages 
installed.

So my question is: it is possible for yum or associated utils to operate 
relative to a package list and architecture that differs from what is 
actually installed on the system?  (All documentation that I have read 
suggests that yum always takes the system's current state.)

Thanks,
Phil