Confirmed: the compound proposed by the author has been made and studied since at least 1992 (https://pubs.acs.org/jmcmar/article-abstract/35/22/4135/7112...). It had activity interesting enough to be published, but like almost all active molecules, it wasn't interesting enough to push into the clinic.

Some other inaccuracies I noticed in the post:

"DXO is more lipophilic, meaning it’s attracted to fats and lipids."

Dextrorphan is more polar, and therefore less, not more, lipophilic than dextromethorphan. It is still able to cross the blood-brain barrier because it is still a fairly lipophilic molecule.

"My first thought was replacing the methoxy group with 3-fluoromethoxy. On paper, this seemed like it could be an effective way to interfere with CYP2D6-mediated demethylation, but synthesizing a fluorinated DXM analogue would be difficult and costly."

There is no such thing as a "3-fluoromethoxy" group. What I think the author meant was "trifluoromethoxy" ("3-" and "tri" have very different meanings in chemical nomenclature). The trifluoromethoxylated derivative of dextrorphan has, in fact, been synthesized and patented (https://pubchem.ncbi.nlm.nih.gov/compound/24997117).

"The structure of DPO is (+)-3-(isopropoxy)-N-methylmorphinan, with the 3-methoxy group of DXM replaced by a 3-isopropoxy group."

The "+" indicates the direction that a chiral nonracemic compound rotates plane-polarized light in solution. While there are computational methods to predict it, it can't be assumed that the derivative of a "+" compound will also be "+". Better to use "R" and "S" (CIP nomenclature), since they are determined entirely by the 3D structure of the chiral compound and don't require a physical measurement to determine.

Dextroproporphan: An Analogue for a Better Dextromethorphan?