First of all, what is actually “blatant” about it 
or why should you feel “shameless” about chipping in?
Secondly, why do you need to mark your place on the thread anyway, when surely you can just read the thread?
Thirdly, can you not really think of anything whatsoever to say on the thread that has intetested you enough that you’re reading it? Even a YABU or YANBU would do. Or, “Good luck OP.” Anything really.
Finally, AIBU to find the word “placemarking” irritating in itself and the whole thing pointless?