Of course you can have a cup holder...
I've come on looking for help with a non-functioning heater in my 55 plate 156, but I couldn't resist replying to this thread first!
I've had my 156 for 3 weeks, and have already made a cup holder, having shared your frustration!
If you cut the rubber coin holder off, next to the handbrake, it leaves a large enough space for a cup holder. I put some fastflex tile adhesive in a plastic bag and sat it there for a day or two to fully set, moulded to the uneven shape of the centre console. I then wrapped a drinks can with a few layers of paper towel, to make it a little larger, and drew around it on my new moulding. With the angle grinder, I simply trimmed my lump of tile adhesive to the right shape and size, and flatted off the top.
Now, with some leatherette wrapped around the can (in a similar finish to the dash) I built up some fibreglass around the base and can - two layers was enoughy - I then folded the leatherette around the outside and secured with very strong double sided tape.
This gives a perfect sized can holder, which sits very securley on the uneven console - with a screw through the bottom for good measure!!!
I'll take a picture and post it up if you want?