I just had to share the happiness with people who'd understand! We just reserved some Gs for an outbound flight LHR to SFO and inbound from LAX to LHR on the exact dates that we needed. The agent I spoke to on the phone was super helpful (and seemed strangely relieved/pleasantly surprised that I knew what an M class upgrade was!) and the tickets are on hold until some miles clear into my account early next week. I'm extra pleased because the price on the tickets is great, saves us quite a bit on PE (which is what we would have booked with cash if we hadn't been able to upgrade). I've been eyeing the availability for a while and praying they wouldn't disappear, but decided today to bite the bullet and put the tickets on hold. We seem to have been very lucky as there's a decent amount of availability around the dates we're looking at. Sadly when we're looking at going to LAX next in early July there's no availability whatsoever... guess we'll be paying for PE that time! But I can't complain; FC miles have been good to us so far 
