My personal opinion; he wasn't doing anything wrong really. I do think it is a tad unprofessional, and I wouldn't really do it (not right opposite a pax anyway!!), but I kinda feel sorry for him that he was innocently just having a bit of humour and bit of fun and this was placed on the internet now for the world (including management to see)
As for sitting in Brace positions on take off landing etc... my airline (a certain large airline operating out of London!) does have a procedure for the silent review, thinking about drills, commands, door operation, checking for hazards, where all your eqpt, alarm etc is near your crew station/door and sitting in ''the semi brace position'' with harness low and tight. It is a good idea though actually, even if some people might laugh, I always get into this position, and do have a think especially if I'm on an aircraft I haven't been on for a while or at a door position I don't often work at etc... Although with this guy he obviously wasn't doing that, whilst it's not the most professional thing, he was having abit of fun, and it wasn't as if he undone his harness or anything.. or did it through the approach...