Ken - Buy a full fare ticket on a credit card and cancel it for a full refund. I'm sure this is much easier than going through a ton of red tape etc etc.
I have flown into the DR many times without a return ticket, I have been lucky and never asked or had a problem.
The DR does not have reciprocal visa agreements like the UK and the USA, so the airline will allow you to board with a one-way ticket.
And a final point

Usually one way tickets are not much cheaper than a return in my experience.