It's really hard not to immediately picture Homer Simpson with a hacksaw and dreams of intercepting the flow. Then I thought... cartoons aside you could probably tap and divert a very small portion of the beer. Then I wondered what kind of threshold any monitoring systems might have which would be alerted to a drop in pressure, and therefore how much beer one could swindle before alerting the BierKops... mit their hops.
Hot Tapping (https://en.wikipedia.org/wiki/Hot_tapping) is actually really easy on poly line - and given the non-hazardous nature of beer I'm guessing they have little to no flow control on the line either. It sounds like the line runs under public cobble streets though so that'd be pretty tough to pull off. The usual hot tap for this sort would be just a steel saddle with a pointed nipple on the inside that's applied with a few U-bolts or similar - certainly something in the range of a backyard shop or garage tinkerer.
The amount of these on gas & gasoline lines in rural areas is actually pretty surprising, and they usually aren't picked up unless there's an incident or they cause a dip in corrosion potentials. Most flow control / leak detection systems won't catch under a 1% difference.
I believe that this year the workers installed more beer pipelines (kinda makes sense, given the terrorism threat and the heavy restriction on vehicular traffic), but can't find any numbers.
Poly pipe is pretty good for being non-porous and difficult for much to grab onto or grow inside - Oil & Gas uses poly lined steel pipes frequently where microbially induced corrosion is an issue (various extremophiles that live in the reservoirs that like to eat iron and shit acid, etc).
At that length I'd be surprised if they didn't - caustic & acid will require a few line flushes etc, whereas a plastic cleaning pig can be run with the beer in line without quality issues provided they run it every few days.
Sorry, I'm a little tired.