I would like to know if Oda has offered any explanation as to what the tattoo on his face signifies/denotes, or if something has been said about it in the anime/manga.
As far as I know Oda sensei has said nothing about it. The tattoo wasn't present in Volume 0 when Roger was executed 24 yrs ago, but present 12 yrs ago when Dragon saved Sabo. So it is clear that Dragon had it after the Age of Exploration. Theories exist as to what it means, judging from this fact.