I've done quite a few one-way bookings as round trips bookings. What I do is is very simple.

I book A-B-A and let the return to a far away date as possible allowed by the ticket. Then when I need to fly, I change the return date to the time I want because you never know if might do that one-way return.

I do this even for restrictive round trip fares; I book the outbound of a second ticket to be the inbound of the first trip and vice versa.