the next buy promo won't be until at least this current offer ends. and finding f availability is very hard. I would secure it now if you can.
Thanks for the advice MEL_Traveller... considering what you have said - and thinking on it a bit further myself - I think I will go for my option #2, namely to use a combo of buy + Amex->SPG->MR to get the required points now.
I would normally think that sharing works both ways... but then last year they had a promo where an account could only benefit once... so you could either share with someone (and they got the bonus) or they could share with you (and you got the bonus). either way, US argued that you, the account holder, beneffitted (either by sharing the bonus miles or receiving them)
This appears to be how the current promo works too, unfortunately. The T&Cs state:
Offer valid for your FIRST Share transaction (purchaser or recipient) made between July 1, 2012 00:00:01 AM PT and July 31, 2012 23:59:59 PM PT. This bonus is only applicable for Share Miles and the offer does not apply to Buy or Gift Miles transactions. Bonus amounts will be awarded upon completion of the transaction. Bonus miles will be awarded to the recipient of the Share. Offer is subject to change. Offer cannot be combined with any other offer. Only purchases made online are eligible.
As usual it's a bit ambiguous, but the "first share transaction (purchaser or recipient)" seems to suggest we can't mutually share - and as I said earlier it would be expensive to try to do this and have them fail to award the second lot of bonus points.