As long as your tag-on is less than an hour after your tag-pff, it will count as one trip, so you will be charged Central-Richmond. Tag back on more than an hour later, it will be two journeys. Interesting, as break-of-journey is not currently permissible with suburban tickets
Break of journey might not be allowed on 'point-to-point' tickets, but it IS allowed on Periodicals (I do it all the time). And it's perfectly legal, not just a loop hole in gate-line programming.
In the 'Passenger Fares Handbook' -
c. MyTrain weekly and fortnightly tickets are available for travel between the stations shown thereon for which the fare has been paid. Break of journey is permitted in either direction at any station on journey/route paid for.
There is a similar item in Clause 73 for the other periodicals.
Where there is a choice of route, typically a break-of-journey is only permitted on the shortest route, except if the airport line involved, where break-of-journey is allowed, even if it's not the shortest route between your station pairs.
It would appear that with Opal, all the weekly/periodical tickets will disappear, replaced with the weekly 'trip' cap. Since break of journey is allowed on a ticket type it's replacing, Opal has to allow break of journey.
Now as to how many people make use of the break of journey 'feature' of their periodical, I don't know. I suspect most people think their weekly is exactly like a 'point-to-point' ticket except its for 5 days., when it's really unlimited trips between any station inside the 'end points' of your ticket for 7 days.
I worked out the fare bands applicable to my commute, and buy my periodical for the maximum extent of the band, not for my commute pair. (It's the same price after all). This has the side effect over covering a large portion of all additional train journeys (over my commute) I'm likely to do and also allows me to catch express trains that don't stop at my station and hop off at the station past mine they do stop at. (Which is also a major regional centre, and not that much further walk from my home anyway.)