Hi Lindsay
You will be in Newtonmore, Kingussie or Aviemore the day before Coylum Bridge and you will likely phone in to Control. Get a weather report when you do. At that point you will likely be able to make your decision at that stage and go directly to Glen Feshie if needed.
Foul weather generally takes a day or two to brew up (not always) so you get some warning.
That is basically what I put in my route cards. Decide at Kingussie.
In the worst case you can simply do the long route via Glen Feshie. I've done Kingussie to Linn of Dee in one day when I had to.

Colin

My plan is the same as Colin's. However, I'm not sure if the Lairig an Laoigh to the E is an acceptable FWA as it doesn't get quite as high as the Lairig Ghru. If acceptable, this may be a simpler option if you're already in Coylumbridge...

Re the Lairig an Laoigh, yes and no. The route over the shoulder of Bynack More is fine, with a wide unmissable path, do-able unless conditions are extreme. The Fords of Avon though is the killer, an impassable torrent if in spate, so onward progress to Braemar could be completely blocked. You can still go via Ballater, using the Avon, Builg and Gairn paths.

Hope that helps.

Colin