Any luck with Adobe Flash v. 21 for Safari?

Are you using the .pkg file from Adobe's distribution page? I've always just thrown that into Casper Admin and deployed it with a policy, and it's been working fine for us.

Make sure you're getting it from the enterprise deployment site that is shown above, and not the public Adobe software site.

The email you'll get back from Adobe after they have granted permission to push their apps will look something like this:

*Thank you for submitting a request to distribute Adobe® Software. Your request has been approved, and you may now distribute the Adobe® Software listed below, subject to the terms and conditions set forth in the Adobe Reader and Runtime Software Distribution License Agreement.

Approved Software:
Adobe® AIR® Runtime
Adobe® Flash® Player
Adobe® Acrobat Reader® DC Software
Adobe® Shockwave® Player

Attached is an Adobe Portable Document Format (PDF) file of the distribution license agreement. Please keep this file for your records. The agreement is valid for one year, after that one year you can renew the distribution license agreement by following the same process. Below you will find the necessary details and download links for the product(s) that you have been approved.
*

I haven't had any trouble deploying out the packages available there as is, with no repackaging required.

AutpPkgr will make your life much easier when it comes to getting the latest updates into Casper: autopkgr GitHub page

Someone posted a great script for updating the flash player; I can't recall who it was but it was here on JAMF Nation somewhere. I save the script in /Library/AdminToolBox which I use for such things and just call it periodically. Here is the script:

#!/bin/sh
# This script downloads and installs the latest Flash player for compatible Macs
# Determine OS version
osvers=$(sw_vers -productVersion | awk -F. '{print $2}')
# Determine current major version of Adobe Flash for use
# with the fileURL variable
flash_major_version=`/usr/bin/curl --silent http://fpdownload2.macromedia.com/get/flashplayer/update/current/xml/version_en_mac_pl.xml | cut -d , -f 1 | awk -F" '/update version/{print $NF}'`
# Specify the complete address of the Adobe Flash Player
# disk image

fileURL="http://fpdownload.macromedia.com/get/flashplayer/current/licensing/mac/install_flash_player_"$flash_major_version"_osx_pkg.dmg"
# Specify name of downloaded disk image
flash_dmg="/tmp/flash.dmg"
if [[ ${osvers} -lt 6 ]]; then
  echo "Adobe Flash Player is not available for Mac OS X 10.5.8 or below."
fi
if [[ ${osvers} -ge 6 ]]; then

    # Download the latest Adobe Flash Player software disk image
    /usr/bin/curl --output "$flash_dmg" "$fileURL"
    # Specify a /tmp/flashplayer.XXXX mountpoint for the disk image

    TMPMOUNT=`/usr/bin/mktemp -d /tmp/flashplayer.XXXX`
    # Mount the latest Flash Player disk image to /tmp/flashplayer.XXXX mountpoint

    hdiutil attach "$flash_dmg" -mountpoint "$TMPMOUNT" -nobrowse -noverify -noautoopen
    # Install Adobe Flash Player from the installer package stored inside the disk image
    /usr/sbin/installer -dumplog -verbose -pkg "$(/usr/bin/find $TMPMOUNT -maxdepth 1 ( -iname *.pkg -o -iname *.mpkg ))" -target "/"
    # Clean-up

    # Unmount the Flash Player disk image from /tmp/flashplayer.XXXX

    /usr/bin/hdiutil detach "$TMPMOUNT"

    # Remove the /tmp/flashplayer.XXXX mountpoint

    /bin/rm -rf "$TMPMOUNT"
    # Remove the downloaded disk image
    /bin/rm -rf "$flash_dmg"
fi

Completely agree with @bvrooman That is the way we have been doing it here, no issues to report.