My interpretation of the FAQ page with the 10% comment is that that rule only applies when you make a reward booking before you have all the necessary reward points. That's the wording of the question it's given as an answer to. If by "next weekend" you mean 7 days from now, not tonight, you should be able to purchase the 48,000 points you need, wait the 24-48 hours for them to clear, and then make your reservation. Have you tried calling MR to clarify?

By the way, before you commit to buying the points you should double-check your math. You're proposing to buy 48k points, which at $12.50/1000 is $600. According to the hotel reward chart, 80k points is 5 nights at a cat 4 property. If you paid cash for the whole stay it'd run you about $700 plus tax, and you'd still have your 32k points. Alternatively, you might consider buying just 3k points to get your total up to 35k points, enough for 2 nights at a cat 4. Then pay cash for the other 3 nights. The total out of pocket for that would probably be under $500, AND it would fit under the 10% limit if you really do need to book the reservation before you purchse the points.