Yes you can. But if you tell use for which trains this is (origin, destination, date, departure time), maybe there are alternatives that don't required the reservation to be sent via the post.

Hey

Train informations are:

20 april: zagreb glavni. Kol to split at 3:20 pm 

27 april: Split to Ljubljana at 8:31 am

29 april: Ljubljana to Venezia Mestre at 2:20 

Thanks

I can give you some information about the second and third trip.

The first train to Zagreb needs a Reservation, that you can book via ÖBB? But I’m not sure. I’m abel to book it via ÖBB Online Shop (choose traveller discount Interrail/Eurail).

The Euronight from Zagreb to Ljubljana should run also seats, but I can’t book it via tickets.oebb.at or the ÖBB App. Maybe you don’t need even a reservation and you only have to hop on. 

From Ljubljana to Venezia:

I’m not sure wich train you want to use? At 14:20 (2:20pm) there is no train from Ljubljana.

But at 14:09 (2:09 pm) an Eurocity to Trieste. Here you don’t need a reservation, but you can book it (3€/person/train) via tickets.oebb.at or the ÖBB App. But you need to pay the Eurocity fee for Italy (2,60€) via tickets.oebb.at or the ÖBB App (Choose as discount Interrail/Eurail). 

For the Frecciargento to Venezia at 17:05 you need a reservation and you can book the reservation via  tickets.oebb.at or the ÖBB App (10€). You can choose also to take a regionale veloce to Venezia S. L. DO NOT TAKE THE TRAINS VIA UDINE, it’s longer!!!], and you only have to hop on the train (no reservation needed). Next departures from Trieste are 17:16 and 18:16.

20 april: zagreb glavni. Kol to split at 3:20 pm: since that's on a Wednesday, I don't expect this train to be full. Just book it at a ticket office in Croatia. It's only HRK 8 (~EUR 1.07) per seat.

27 april: Split to Ljubljana at 8:31 am:

Split - Zagreb 8:31: book it when you book the reservation for 20 April.

Zagreb-Ljubljana 18:40: not sure if reservation is really mandatory, especially in the coach Zagreb - Villach. But if you want a reservation, just book it at the ticket office with the above 2.

29 april: Ljubljana to Venezia Mestre at 2:20:

I see the app says EC 134 leaves Ljubljana at 14:20 but this seems to be outdated since both DB and ÖBB now say 14:09. This is why you should not use the app to plan… the app will get an update eventually.

Anyway, there are basically 2 options. 1st option:

EC 134 has optional reservation. You can get one for 3€ via ÖBB (choose "Seat only”) but I don't think it's necessary.

The Frecciarossa Trieste - Venice has a mandatory reservation of 10€ that's best booked via ÖBB using these steps.

Option 2:

EC 134 has optional reservations (see above); Trieste - Venice is a regional train and no reservation is needed.

Don’t forget the Pass Supplement for the Eurocity in Italy. It cost 2,60€ online via tickets.oebb.at or the ÖBB App (Choose as discount Interrail/Eurail). Maybe also bookable onboard of the train. 

Thank you for all these valuable answers, it helped a lot! I made reservations via OBB, great site!

I realise there are quiet a few things I still have to find out. 

Another question, probably the most silly one ever asked: what happens when you swipe the button to the right? When you need to do that? Just before you hop on the train?

Exactly. You need to do that before boarding; it will add the journey to your pass. If you then need to activate a travel day, it will also offer to do that. After adding a journey to your pass, you have a QR code to show for ticket inspection. Sometimes, you already need it to enter the station or platform, e.g. in some stations in the Netherlands, Italy, France, Spain or GB.

You may also want to check the mobile pass FAQ.