My concern for you would be that you would launch OK at 8:30am but when you come to recover the slipway would be busy. cars and trailers would be blocked in, people all over the place etc etc.
Trouble is that if you get "stuck" on the Thames and miss your recovery slot with tides etc you've got a bloody long time to wait to recover and not much option in terms of places to moor whilst you're waiting!
If you read some of the blurb on the website it mentions about parking restrictions on Putney embankment and requiring permits etc etc. http://www.wehorr.org/note-crews.html
Alternative launching sites - only ones I personally know of are in Docklands so not much use to you all coming from the West. As I say I'll be launching nr Woolwich so wont have the problem, just thinking of you guys!
Is the rowing event on Sunday as large? If not might be worthwhile rescheduling if people can make that. HT is an hour later which gives you either another hour or two on the water or an extra hour in bed! (& the weather forecast for Sunday is better - 10% chance of rain as opposed to 40%!).
I guess the final call is upto the guys travelling the furthest.
Let us know!