The Ford Tech Service Bulletin 46/2008 covering turbo replacement recommends doing a 20 min flush with Wurth engine cleaner, but I asked a Ford technician and he said they definitely wouldn't flush. Could be the TSB is out of date, but flushes definitely divide opinion. Personally I'd play it safe and not bother.
I would say disconnecting the feed pipe is a bad idea, you're going to be starving the turbo bearings of oil, and you're going to have a job doing it with the cat section of the exhaust in place. Ford recommend disconnecting the oil return line, replacing it with a long tube, and then letting the engine idle for 60 seconds while collecting the oil in a container. You should get 0.3 litre or more of uninterrupted flow after running this test. I guess you could remove your feed pipe and probe it a bit to see if its gummed up.
One other thing, if your car has a DPF you'll need to pop the radiator fan and shroud out to gain access.
I'm just putting a shopping list together for my local Ford dealer, some of the bits are a bit expensive, I think a new feed pipe is at least £50, so it could get a bit pricey for preventative maintenance, but a lot cheaper than having to replace a turbo etc. ... I'm going to clean and reuse the sump pick up pipe but replace most other bits. If I have a successful story to tell I'll stick some info back here....