Its standard in journey planners and ticket vending machines for other operators to put a "via" option in the menu. it shouldn´t be too big a deal to to set up.
Its not just useful for alternate routings (in the few cases they are available herre) but also for small detours off a direct journey.
Eg If I was traveling Cork to Dublin but had some business in Limerick it should be possible to buy a ticket that says Cork-Dublin via Limerick and I could the travel Cork to Limerick and then onwards to Dublin.
But back to my original question how is it possible for me to book a ticket on my preferred trains?
surely this cant be an excluded combination or something silly like that.