I think it is a great idea and could be extended to other books in the series. Only three so far have been made by Walden, and all three were also made for the BBC television series that also made Silver Chair. However, all 7 Narnia books came out in both the BBC and FotF audio dramas. It is nice that those series are complete.
On the whole, I chose the two audio dramas in preference to the TV program SC. I fear that the television series in 6 episodes loses something when I attempt to watch it in its entirety, especially as it takes longer to watch it than a comparable Narnia film, such as the three Walden ones. This is an advantage with the radio series that they can be more easily listened to whilst driving, ironing or something like that, whereas to watch a film or even television, even in hourly weekly programming, needs more of one's time and undivided attention.
Some things I liked about the BBC television production of SC include the fidelity to the book, the owls, despite the clunky costuming, in particular Warwick Davis' Glimfeather, and the appearance of the added-in dragon on Ettinsmoor, all ready for some dragon-to-dragon socialising with Eustace. I preferred Camilla Powers' performance as Jill and David Thwaites' performance as Eustace to that of the four Pevensie actors in that series. Tom Baker, a former Dr Who, put in a sterling performance as Puddleglum, though in that movie he injured his hand rather than his foot. But I liked in particular, the scenery for the Underground palace, with that stained glass effect and also the way they did the Silver Chair itself.
Some things I disliked about that BBC version were the use of the PC Caspian, a little boy, rather than BBC VDT's Samuel West as the resurrected Caspian, which I feel would have been a better choice. Some of the special effects were clunky and outdated. And though Barbara Kellerman made a good LOTGK in SC, I couldn't see her going back and playing Jadis afterwards in a MN production. Lastly, I'm not sure how BBC's Rilian would translate to a Walden movie.
I'm not sure which of the two audio drama productions of SC I prefer yet. They are both interesting but completely different. FotF is prefaced with some valuable commentary from Douglas Gresham, and a distinctly scoundrelly LOTGK snippet. Whereas the BBC audio drama starts with LOTGK making some sort of vow. Then Eustace discusses his school with Lucy and Edmund whilst they wonder what is happening in Narnia. This is the BBC radio drama's chance to portray the back story of how Caspian and Liliandil got married, the arrival of Rilian. Trumpkin and Drinian's fireside discussion then goes straight into the tragedy of how Caspian loses his bride and heir. And then we have Jill and Eustace at Experiment House, off to Narnia to solve the situation.
The FotF audio drama, by contrast, sticks more closely to the book order. Most of the back story of Caspian, Rilian and Liliandil is told via the narrator, at the Owl Parliament. I like the narrator for the FotF who is used for good effect. Whereas the BBC one relies more on the characters themselves. However, I do prefer the BBC Owl parliament to that of the FotF. They sound more like owls, and the BBC Puddleglum sounds more authentic than th e FotF one. Just my tuppence worth.
I voted for the Focus on the Family and the BBC TV versions because they were really faithful to the book. 
