Like Johanna I have my own in-house Victor Meldrew. I have tried all the strategies listed above, which have not worked. Undertaking a 'project' or being involved with a group just brings more things to complain about into his repetoire. I have encouraged him to talk more with our lonely elderly male neighbour, but as he (the neighbour) is deaf, the conversations are pretty one-sided and DH acquires a whole new batch of things to complain to me about.
I did find out the complaint line numbers for the major TV companies and can direct him there if current moan concerns re-rescheduling his favourite programmes, but did not have same success with urging him to write 'letters to the editor'. He thought that was a good idea but wanted me to write the letters - He said 'You write better letters than I do': I said 'but I'm not complaining': He said 'but you said you agreed with me and I should write to the paper'.
What we need is a Grumpsnet with Topics such as Young People Today, Vanishing Tradesmen, The PM, Chancellor, Mayor, etc. is misguided, ill informed, off his trolley, IN MY DAY...' How I taught my wife how to do housework or shopping and perhaps a page for photos of their DGC who of course never do anything wrong (in spite of their parents bringing them up in the most peculiar way).
Or do you think such a site would just giving them more opportunities to rant on 'You'll never believe what this chap said on Grumpsnet today'?