This is a good discussion, thanks for bringing it up.
I'll start by pointing out that that article was published way back in 1996 and there has been a lot more research in this area, both sanctioned and underground, in the intervening years. I do expect there is some relevance to the studies Ott pointed out where serotonin levels were artificially inflated via use of MAOI's for days prior to the administration of the 5-ht acting psychedelics. It is oft reported that people on SSRI's need much higher doses of things like mushrooms and LSD to get the same level of effects as people who are not on SSRI's, and this is very likely the result of increased serotonin levels. I'm not aware of any research specifically on endogenous serotonin levels and how they effect the binding affinity of 5-ht acting psychedelics, but I'll dig around a bit on google scholar and report anything I find.
I think the use of changa is good to look at as a case study for this issue. It's well documented that changa (as well as pre-dosing oral harmalas) significantly extends the duration of action of smoked DMT. If not outright potentiation, the breakdown/removal of the DMT is certainly being effected by the inclusion of the beta-carbolines.
At this point, contrary to the handful of reports Ott had to go in in 1996, there are thousands of reports of "psilohuasca" posted online and they consistently report much stronger effects than those of the mushrooms alone. As someone who has taken measured doses of mushrooms from the same batch at the same doses on multiple occasions with and without harmalas, there is no question to my mind that there is strong potentiation between harmalas and psilocybin.
You also have to take timing into account, if the studies Ott cited had administered the MAOI's at night rather than during the day, it's likely the results would have been quite different. In addition there are the factors of MAO-A and tryptamines and MAO-B and phenethylamines. Dosage is another important factor, taking just enough harmala to "activate" the DMT lends to quite different results than taking doses large enough to fully inhibit MAO.
There is the curious factor that if you add too much harmala to the equation, it tends to dull and ground out the experience quite a bit. I suspect that data is relevant to this discussion as well, though I'm not entirely sure exactly where it fits in. I am inclined to suspect that the GABA effects of harmalas also play an important role in the pharmacology here, but again would need to do moar research to pinpoint exactly what that role may be.
In short, there is a lot going on with these interactions, and a lot moar research is necessary before we can attempt to make any definitive statements, beyond mere anecdotes, at this point in time.
Thanks again for bringing up this discussion, I'm interested to see what kind of research and insights the community can offer in regards to this question.