on the 124 the lock shouldn't be attached to the handle - it's a seperate part which comprises of the lock and the plastic surround which, reading between the lines, I guess is the part you are trying to use with your old lock mechanism to get the chrome surround.
I don't think there is an easy way, that plastic plate is moulded on. You can't get the insert out without the key as you need to have the tumblers retracted so drilling may be the only way. Putting the current lock mechanism into your new handles won't be easy either as the tumblers have to line up exactly and getting that retaining pin back in is a real nightmare.
Drilling is also dangerous as you may damage the lock barrel housing - make sure any drill you use is very sharp and slow turning. If possible get it on a bench drill so you can accurately control just how deep you go in and at what angle.
If the worst comes to the worst you can ask your dealer to get you a new lock mechanism for the doors and tell them that you want the ones with the chrome inserts. MB will supply either for the W124.
Sorry I can't be more help
Andy