For a list of all of the files that are installed, you can check in
the .list files located in /var/lib/dpkg/info. Note: the .list files
also contain directory names, so it is not a list of files only in
terms of absolute pathnames from the '/' directory.

If you know the date when an update has been applied, you can simply
do an ls -lt *.list command to list the .list files that have been
applied since a certain date.

One thing you can then do, is construct a script to filter out the
directory names leaving only absolute path names of all of the files
in a particular .list and combine the results into a file.

Then with that combined file, assuming the update was incremental for
a particular piece of software, you could do the following as root:
# cd /
# tar -cf /root/update.tar `cat combined-files.list`

The tar command would feed all of the combined files absolute
pathnames from the updated files combined .list, as input into the
tarball file. Then you could compress the tarball with either gzip or
bzip2 to get respectively, either update.tar.gz or update.tar.bz2.
Note: ignore the fact that tar will drop the leading '/' in an output
message from the tar command.

Then you would simply need to transport the compressed file over to
the host that cannot be put onto the network, and then uncompress, and
then extract with the tar command, again as root, from the '/' directory.