What is it that you're making? I don't have much experience with spelt, other than as a secondary ingredient in yeast breads, but I noticed this article by Alice Medrich, who has a ton of experience using alternative flours: https://food52.com/blog... Based on this, one could easily conclude that it's worth a try.
Also, it stands to reason that spelt would work just fine, given that pastry flour is generally a low gluten flour, while spelt, from what I have read, has gluten that is weaker than wheat flour, i.e., it does not create strong, stretchy strands, meaning spelt is likely to produce a more tender baked good. The WW flour will have a different flavor, but it's certainly worth trying the spelt!
I'll be interested to see others' answers to this. ;o)

I just baked a blueberry galette with a spelt crust. I can't tell you how it works as compared to WW pastry flour but I can tell you that the dough was very tender and cracked easily when rolling out and the finished dough was crumbly. It tasted good but leaked a lot in the oven where cracks appeared.

Spelt flour is not necessarily made from the whole grain--you may end up with a different texture or need to use less liquid.

More on substituting spelt flour for whole wheat (regular, not pastry) here: http://www.thekitchn.com... and on baking with regular pastry flour and WW pastry flour, from Bob's Red Mill: http://www.bobsredmill...
Still, would love to know what the recipe is for, as one must consider these questions in context. ;o)

Sorry for omitting what I was preparing to use the spelt flour with. I have a zucchini quick bread recipe from Bob's Red Mill.