Hey, ÖBB can’t sell for this train the reservation. It should be possible at ticket counter of MAV Start in Hungary. Maybe it could be also sold by CD ticket office in Prague or ÖBB ticket office in Vienna. 

And yes it is a bit risky, but you could think about to travel to Zagreb with a day train and then from there on a nighttrain to Split (but this is also risky, because, the nighttrain from Zagreb to Split also needs a reservation.

You could ask directly MAV Start the train company that runs this train, how you can buy this reservation from outside Hungary. 

Gracias @angelo , puedo intentar comprarlo el día 13 que llegó a Praga o el 16 que llego a Viena. ¿Serían suficientes días de antelación?

Para el resto de mi itinerario no voy a coger reserva, las voy a evitar. Según mi recorrido.... ¿Lo ves factible?

The rest is without reservation, except Barcelona-Prague (no direct trains, and also reservation compulsory in Spain and France on high speed trains)