HA is pretty much the same, WB the same, logic says to move the saddle back to the same effective place lengthening the cockpit and he'll be in the same place on the same WB....or is that too obvious?
While a greater setback post on the small frame can give the same cockpit length you are changing the saddle/ pedal relationship which IMO should be fixed in the correct biomechanical position and reach only adjusted with different stems. However people move their saddles all over the place and don't die so whatever works for you I guess
Also with a different ETT you change the weight distribution over the bike so the handling riding out of the saddle in particular may feel a bit off.
I don't know if any of these things might be noticeable but are the issues to consider IMO