Unless I am missunderstanding your use of closed loop system, this refers to using a pump to take water from one area of the tank and return it to another. I actually did mention this type of circulation system in my articles though most of the time I was refering to them as internal circulation systems. And while most of the time I was refering to power heads, I also gave an example for head calculation using an external pump in this configuration.
Having said all this I hartily endorse use of such a closed loop system, if you recall my recommended configuration for all but the smallest systems was a combination of internal and external circulation systems. The closed loop system has the advantage of little or no head to work into so it can potentially be a smaller pump for a given flow rate requirement which is desirable from cost, noise and heat perspectives.
As to using wave makers on external pumps - I've done it for many years but you need to make sure of two things before you do this yourself. One, can a given wavemaker handle the desired pump - pumps have certain power requirements in terms of amperage requirements and wavemakers use electrical switches of some sort (normally relays, either mechanical or solid state) and these switches have limitations to the amount of current and voltage they can reliably handle. If you exceed this switches rating by tying to operate too large a pump you will likely cause the wavemaker to fail. The second question you have to ask is will the pump in question reliably turn off and on repeatily, for some pumps the process of turing off and on is more stressful and in these cases use with a wavemaker is not recommended. Unfortunatly the only way I know of telling whether a pump can reliably run on a properly size wavemaker is to try it or know someone else who already has (the maufacture may be able to tell you as well but as most larger external pumps used in aquariums are not specifically designed for this use they may not know).
In my own personal experience I have been able to successfully use Iwaki 30s and 40s on wavemakers for years as well as MAK4s, on the other hand I blew up several larger Rios (2500's) in similar fashion and have had more than one person tell me that Ehime hobby pumps also have difficulties on wavemakers. Again, the best advice I can give is to ask around and see if someone else has done your homework for you and already demonstrated success with a given model of pump on a wavemaker.
Hope this helps, RN