Never mind Hew about how legal it is for ATC staff to fly, in my case I was worried that the first officer was on an almost equal position to me! Some 12 years ago I managed to bag a trip on a DC6 to some exotic Caribbean island; on return to base P1 had decided he had a long day and asked the FEng if 'the British guy in the jumpseat' wanted to have a go.
What could I say - 40 minutes in the left hand seat of a REAL aeroplane over those crystal blue waters. I've managed to get a few good trips in my time, but that was the right up next to dogfighting with a Sea Harrier whilst sat in the back of the Boscombe Comet 'Canopus'....
On the subject of fam flights, I seem to remember some ex-Heathrow colleagues (John MacDermot, Derek Harriss RIP and Terry Quantrill I think) getting a ride to Budapest with Malev. The shock came when the captain gladly handed the RTF over to Derek for the entire journey!
And lastlly, there was an 'ATCO scam' at Heathrow whenever anyone had a fam flight with MEA to Beirut; it was deemed by some wag as a good bet that they wouldn't return and took out a (long since removed) short term travel insurance policy from a hole in the wall in T2!