hello: Once the OU adds “D” to the label of the the muffins, it is no longer likely that a frum Jew would eat them with meat, and there is no issue of Bittul Issur.
Well the chavos da’as says that putting siman after the baking doesn’t help as the pas is already ossur, and making a siman only works before the bread is finish being baked. I saw this in other achronim too.
You can argue that he was talking about a case where the siman may be forgotten to be made (he really was indeed.) And in our case where the kneading was done in mind to make a siman after the baking, where the siman will definitely be made, even the chavos da’as may agree with you.
DY: I just noticed you response to hello that an after the fact siman doesn’t help. I am also wondering why he switched gears so quickly. Maybe its an old letter and he changed his mind.