I'll be riding down to Bethersden for the Fairies 300 for a 500km outing. I guess I could find another 100km from somewhere.
The argument against is that I wouldn't be committed to a 600km 'all or nothing' outing. Then again, 600km is 600km. One for greater minds than mine.
It's an interesting one. A few years ago I entered a 100km ride with the intention of cycling 100km home afterwards and entered the whole thing as a DIY 200 (before the days of ECE). On the day I was so hungover I barely scraped round the 100km ride and so ditched any idea of cycling home, but still submitted the 100km ride Brevet Card (no points except for FWC points).
I'm not sure what I would have done if it had been worth real AUK points. I guess I didn't complete what I set out to do and so I should get nothing unless there's an opportunity to 'reset' what you're doing, i.e.
enter it as ECE 200+Cal300+100 then the various bailing options, and outcomes, are:-
1) Bail on the first 200 and you can get yourself to the calendar event and just ride it as a 300. OK since you can DNF the whole thing and submit a new goal of just the calendar ride (before you've started it).
2) Bail on the calendar event or the ride home and you're stuffed as you've already committed to doing the whole thing. The problem is that the Brevet card for the calendar ride has already been submitted.
Enter it as ECE 300+Cal300 and you can:
1) Bail on the first 300 and get yourself to the calendar event to ride it as a 300.
2) Bail on the calendar event and get nothing for the day(s) riding.
It's a fast 300, and very fixed friendly. With the 2AM Sunday morning start you could do all the extra distance (minus the ride home) up front.
What a silly idea!
FWIW Scottlington and I did the equivalent of ECE-ing the Fairies 300 into a 600 back in 2008. Starting 6am on Saturday we bimbled from Putney up to Cambridge and back for 200km, then did Putney, Royal Tunbridge Wells, Bethersden for another 100km arriving at about 1am (not as early as I'd hoped). Tried to grab some rest in the hall before the start, and then set off for the 300 (where the vast majority of the field dropped us within the first few km).
If you do it this way (300km to the start of the ride) then you can finish in time for the train back from Pluckley to London (last direct one is just after 10pm, which should be easy for a flat 300), then it's just a short bimble for you back to West London.
Front-loading a (300km or longer) calendar event can be a good way of guaranteeing a quiet day on your own.