I purchase "open" tickets all the time when abroad.

All these are, effectively, are full-fare Y (or or J or F) tickets, that gives you leverage in confirming a seat on the date on which you wish to travel.

The technicality is that on the tickets, the open segments will say "OPN" under the date.

So it sounds like your question has to do with means and ends - your end is to give yourself leeway in a return date, for which a full-fare ticket (or other fare basis that allows for confirmed changes) is the means.