Meriwether Lewis' great uncle Fielding Lewis was married to Betty Washington the sister of George Washington. Then George Washington's step granddaughter was married to General Robert E Lee.
So the general family of Meriwether Lewis was Washington, Lewis, Lee and Spotswood since Gov Alexander Spotswood was Robert E Lee's great great grandfather through the Carter/Burwell/Morgan/Cooke/Myers family of Millwood VA.
If indeed Meriwether Lewis had Mulatto descendants, they most likely would be connected to the extended family of Sally Hemings and to the Jennings family connected to President James Madison of Culpeper.
You also might have relatives in Pittsburgh where the Lewis and Clark Expedition started.
So if your family were known to be part of the inside servants or highly skilled servants of James Madison, what your saying could be true.