You do bring up a good point however it could still have a good flavor.
I’m just having a hard time figuring a relatively simple way to add both flavors while keeping head retention as well
Aside from what it looks like an expensive gimmicky gift item in a fancy bottle, it depends on how much "smoke" flavor it contains, and how much of that flavor will carry through in 5 gallons of beer. 250ml of maple syrup itself is not enough to add much, if any detectable flavor to a porter, even if added to the bottling bucket and used as part of the priming sugars or added to a keg. I don't mind a light smokey beer at times, but do you want a whole batch of it?
I think adding flavorings to individual glasses of beer, whenever you feel like it a change, is perfectly fine.
Plenty of methods have been presented. I think, roasting the pecans somewhat, chopping them, blot the oils off, then making an extract with vodka is the best method. Either add to a glass on demand, or add to your whole batch at packaging time. If you add it to your whole batch I'd use 2 pounds for the extract. If the beer suffers some lower head retention so be it. The pound of flaked barley in your grist helps to increase foam stability. Maybe add an extra pound of it for good measure.
Do some searches and keep reading to get ideas on what the best tactics are. You've got a few opinions here, I doubt you won't find hundreds more.
Most pro (craft) breweries use extracts, and not the cheapest. One brewer mentioned he uses raspberry extract that runs $300 a gallon or so. The beer (not a sour) tasted delicious, possibly better than when using real fruit, and without the mess of dealing with a ton of fruit pulp in the fermentor.