WW Pastry Flour

Yes, you can. And muffins are probably the ideal case for such a swap. The pastry designation of the flour implies a lower protein (gluten/toughness) content which is better for tenderness in pastries and quickbreads. The only small adjustment I envision is a slight decrease in liquid as the WW pastry flour will absorb slightly less than the standard WW flour that your recipe calls for. But that may be negligible. Just use your judgement when mixing.