Custis went back to the University of Pennsylvania, got his medical degree in 1807, went to New Bern, North Carolina, married the daughter of a fellow physician, set up a medical practice, named a son Linnaeus, and never seems to have practiced natural history again. He lived until 1842--he was the longest-lived of all the people involved in this expedition, and never seems to have gathered any fame from his participation in it.
Thomas Freeman lived until 1821. Jefferson made him, in fact, surveyor general of all the lands south of Tennessee. That was a position he held until his death in 1821. And Jefferson was going to send Freeman up the Arkansas River in 1807. He probably was going to send a naturalist along, too, but we don't know for certain, because Congress, with the result of the Red River Expedition fresh in their minds, refused to appropriate money for an 1807 examination of the Arkansas River, and so Freeman never got to explore on the Arkansas.
The Expedition today is, as I said, almost an unknown one. In fact, there is today not a single historical marker in Louisiana, Arkansas Texas, or Oklahoma, to indicate that this expedition ever actually took place.