Hello Daniel,
after some research i found out that it is possible to deploy older components of DP by simply using csetup.exe in the folder "WINDOWS_OTHER\i386" of a data medium with DP6.20 in it.
You can select "user-interface" and "MS Exchange Integration" along with the DNS name of the cell server.
When you start the user interface it reports that it is not supported to run against a newer version of DP but this is no problem.
I'm now able again to use PowerControls with this old 6.20 client.
mfg
Matthias