I did this today - 40000 miles short of the 120000 required for 3 x SQ IST-SIN J redemptions for August. I bought the extra miles yesterday and called to explain I was concerned that they wouldn't post in time and I'd lose the flights (they have a hard hold limit of 24 hours with SQ).
I agreed to mail over receipts for the Boost and additional mile purchases - the agent then manually added the miles to my account so the booking could complete, with the intention of removing them at the end of the week - by which time, my miles will have posted.
Awesome service really - not sure if this is only on partner awards where there are less IT limitations.