This is from the BJCP guidelines for 30A Spice Herb Vegetable
"This category explicitly includes all culinary spices, herbs, and
vegetables, as well as nuts (anything with ‘nut’ in the name, including coconut), chile peppers, coffee, chocolate, spruce tips, rose hips, hibiscus, fruit peels/zest (but not juice), rhubarb, and the like. "
So that is the "correct" category to enter it in. If the pecan notes are so subtle it just tastes like a Porter than you can enter it into American Porter, but likely the flavor profile will be "off". Experiemental Beer is for a beer that does not fit anywhere else. With the new guidelines it's almost hard to to make a beer that doesn't fit.
I would enter it as a Spice Herb Veg personally.