Are you using American Express Membership Rewards because you have a corporate card provided by your employer, or would you be able to change to another card program that would provide a wider range of redemption options?

Starwood Preferred Guest offers a credit card, which happens to also be issued by American Express, and points can be trasferred to most airline programs, except CO or UA, on 1:1 basis, plus if you transfer 20,000 points you receive 25,000 miles which means the 25% bonus is an ongoing feature rather than an intermittent promotion. This would allow you to accumulate points at Starwood to transfer directly to Flying Blue or to AS, DL, NW at time of redemption depending on who has the award availability.