Yum now supports some rpm transaction flags, either by "tsflags" config 
option, or by "yum-tsflags" plugin.

Currently only partial list of possible TS flags is supported: 
"noscripts", "notriggers", "nodocs", "test" and "repackage".

I would like to propose to add "justdb" flag as well.

The "justdb" flag is useful in combination with "--installroot=DIR" and
"keepcache=1".

After the "yum --installroot=DIR --tsflags=justdb update", a separate rpm
database (DIR/var/lib/rpm) we be updated (with all transactions check etc.).
Then we catch the downloaded packages (from DIR/var/cache/yum) and move them
either to a local repository (in a case where the rpm database reflects some
local partial repository snapshot), or move to removable media and then update a
computer without network access, etc.