I'm looking for a way to remote-control my old Technics Tape Deck
(RS-T330R) and CD-Player (SL-P110). Both of them seem to be designed
to get their RC commands via some cable - and both of them use a
different system.

The best option for me would be a stand-alone RC receiving device
which gets its signals from some remote control and is connected to
the CD-Player and the Tape Deck using their corresponding cables...
(The perfect solution would be if I could use my Panasonic Receiver's
Remote Control)

You can find pictures of the connectors at
http://stefan.desire.ch/technics/

Looks like you have two different Technics lines on your hands there. 
The one on the left, with the 1/8" plug was a unit that was sold as a
separate component, I am guessing that is the CD?  The one on the
right was part of a Technics rack system, most likely.  They used a
different logic cable to run all IR commands through the control amp. 
I don't know of any manufacturer's products that would interface the
two different systems.

Not a researcher, but this is what I know from working with that
equipment for several years.