We've done this a few times, Hil, but only in PE. It's definitely been worthwhile for us. For example, this October we'd been looking at around £750 each for Economy. I couldn't bring myself to pay that, so we started considering other options. We were low on points having wiped ourselves out the previous year by booking free Reward flights. It occurred to us that, whilst neither of us had enough miles to book a Companion round-trip, we did have enough for one leg each. We ended up paying a total of £1300, including taxes, for 2 PE flights.
There's now an option to book Companion flights online, so it's easy to price up and compare. The number of these special category fares is very limited, especially on the Orlando route. For example, they often only allocate two in PE across the entire reward fares range (Reward, Companion, Upgrade). Consequently, it helps if you're able to book early and be flexible with dates. Several times, we haven't been able to secure our first choice of dates, but with perserverance and a willing Flying Club agent, we've been able to get the discounted fares.
