SMS wrote:

Could I book the return, and my free miles flight, use the
outward then just not use the return I paid for?
Or would Continental suss something was up here and disallow it?

There is no requirement to use the return flight.

You need to read the original question more carefully.

We all know that a transportation company can't force you onto (or into)
their vehicles just because you bought a ticket.

The question is - can an airline retro-actively change your ticket
price (and hence put an additional charge on your credit card) when you
fail to make your return flight as specified on your return ticket? The
additional charge being the difference in cost between a one-way vs
round-trip itinerary.

The fine print on many tickets seems to indicate they have taken this
scenario into account and they claim it as their right to do so (to
charge you).

The question remains - do they *really* do this - at least on a regular
or systematic basis?

You might want to call and tell them, once you reach your destination,
that you are staying longer than you had planned and will use the
return some other time and pay the change fee.

Many people in this situation (and I believe the OP as well) are really
only seeking a 1-way ticket, but they notice that a return ticket costs
less (sometimes much less) and so they buy the return ticket with no
intention of using the return flight. So your advice in that situation
is meaningless.

I would even argue that it's best NOT to call the airline after your
out-bound trip is completed and say that you're not going to make the
return flight, because by telling this directly to an agent (who is
sitting at a computer looking at your booking) it's obvious that you run
an extreme risk of them pressing the right keys that WILL result in your
itinerary being converted into a 1-way ticket and being billed
accordingly.

You've got nothing to gain by making that call.