(Is it just me or is that the most awkward sentence of the last few months?)
Apparently you've not read various posts I've made here and at other forums. You'd really have to try to get more convoluted than some sentences I've written recently.

As for your question, here is my take one the matter.

One hundred and forty or so years ago, one of your ancestors was asked to give his last name. Not uncommon for a Freedman to take the name of his former master, especially if that master was worthy of claiming kinship.

In addition to making use of their labor, slaveholders gave their slaves protection from outsiders, and cared for physical needs. This is not unlike the compact between Laird and clansmen in that matter. The clansmen gave loyalty, and labor to the Laird, and followed the Laird into battle. In return, the Laird protected the clansmen form outside forces, and made sure that they had a roof over their heads and some food in the pot.

Therefore, I'd say you have just as much right to wear the tartan as any other sept of any clan.

BTW, that is a very handsome tartan. What clan does it represent? What does the clan badge look like?