It sounds like a pretty unique situation and that is probably why you haven't found many recommendations.
It is usually best to match the front three speakers. Surrounds are less important and the subwoofer doesn't matter at all.
The question you have to ask yourself is how much time/effort/money are you willing to invest in the project. As far as I am concerned, it's the English adage 'in for a penny, in for a pound'. Meaning if you really want an in-wall solution you should go for the whole solution and do what is necessary to mount the matching center in the wall - but it do it Right, no hack jobs you may have to revisit later.
Now having said that, MY preference would be to just go for an external center. If you can find one from the same manufacturer it may be similar enough to the in-wall left and right speakers to still sound good.
I was considering using the pre-wired in-ceiling speaker locations in my house but as soon as I got in the attic and saw what a major hassle it would be (not quite as difficult as what you have described though), I quickly abandoned the idea, at least for the near future.