Snoopy you should have the basic TPMS already there you just need to enable it via VCDS.
Forgive my ignorance. You say a dealer can enable it without the full kit. Will it then show on the MFD if there is a deflation?
You will hear the beep and the flat tyre light will be lit it will show nothing on the MFD. Now they are quite sensitive I have one tyre that drops 3 psi after about 3 or 4 weeks and the light comes on so a visual check isn't much use for 3 psi. Obviously a visual check is the first thing you do just in case you have a fast puncture.
I just added then link to retrofiting the indirect tyre pressure monitoring system.
http://www.vwwatercooled.org.au/f112/how-retrofit-indirect-tire-pressure-monitoring-5-minutes-7k6-50164.htmlYou will need the Set button in the glovebox to recalibrate it otherwise you have to use the VCDS to do it.
Pushbutton switch for tyre pressure warning centre console black/white
1k0 927 121 C REH - £15.45
or for glovebox switch
1K0 927 121 B REH - £15.45
but I don't know how you get the switch loom?