I know someone who was diagnosed with DVT in the UK. She developed excruciating cramps after a very long haul flight. She was chemically tested at a leading NHS hospital. The test proved positive for DVT. She was put on some nourishing supplement of rat poison for a while; rat poison in that she had to take Warfarin to dilute the blood.
Now whenever she flies long haul she injects herself with a little drop of something which helps prevent DVT. She wears special socks and tries to walk around the economy cabin on a long haul flight at least every three to four hours.
Part of the problem or so I am led to believe, is that if a clot returns, she'll be on blood thinners for life. This apparently is not a terribly good medication for the arterial walls and can lead to pulmonary embolisms which I believe can be spectacularly messy and always fatal.
I have not written this screed above with any great attention to medical detail because I'm not a doctor but it might help explain why maybe you shouldn't be let anywhere near any cockpit if you suffer from DVT.
I've never known of a pilot suffer from this ailment but I can assure you that no pilot seat or cockpit in which I have spent long hours has ever been as cramped as an economy seat on any airline on which I've ever flown as a passenger.