I can't speak on adding them to the mash, but I've tossed my own candied pecans and almonds into secondary for a cider in the past. When it came time to add into secondary, I crushed them and let it sit in vodka for a little bit to sanitize (same day I added it). Then I dumped the entire thing into the batch, including the vodka since it took some of the flavor. I added 4oz pecans/almonds to 3 gallons of finished cider and got good results. I only added enough vodka to cover the nuts.
As for adding different adjuncts to the mash, the most I've done is a pound of pumpkin puree and had subtle results (which for me was great since I hate pumpkin ales).
If your nut brown has been successful, I would brew it as you normally would and try adding the pecans in secondary. I'm assuming you want to do a variance of your nut brown and not make something different, right? Adding it in secondary would also allow for more control by adding it to your finished brown. But you also said you had numerous reasons for adding it to the mash. Nothing wrong with experimenting.
I had heard long ago from a former brewer that Three Floyd's used to add coffee to their mash (might still do it for their coffee beers) and he said it was very acrid compared to other methods of implementing coffee into beer. All perspective I guess.
