Thank you everyone for the feedback. I was tied up most of this past weekend so I wasn't able to do much additional research on this (looking for other examples, studying the rest of the group, etc.) as I originally had hoped. And while I can't definitively say what it is or isn't, at least to my eyes, with it in hand, it does not look like writing.
So my plan at this point is to send it out this weekend to get graded with a note/letter that asks them to consider adding it as a variation if in fact it was done during the printing process and isn't writing. I'll obviously include a link to the LOTG copy and any others I can find between now and the end of the week. I figure it can't hurt to ask, and if they're going to recognize the presence of a white triangle on a 1988 Topps McGwire card, they should surely recognize a prominent black marking on the chest of 1909 Plank card, shouldn't they??
And Jeremy - good pickup on the black markings/tree tops. The registration and black printing on my copy is definitely worse than the LOTG one. I'm guessing that's all it is, but that is a guess.
Lastly, if anyone is interested on where this card came from it was in this lot:
http://www.ebay.com/itm/LOT-OF-9-VIN...vip=true&rt=nc
I will follow up once I hear back on the grading - and thank you all again!