I’m afraid we don’t have any insider information beyond what you have already posted. GW is rather successful in keeping things for themselves (if you ignore these 4chan 40K release schedule leaks although I would not be too surprised if they were purposefully ‘leaked’). You can check the Horns of Hashut and hobgrot release threads for forum member speculations (mainly about nudism and missing appendages though, so be warned).
As for made-to-order, GW does not seem to take it too seriously. Bretonnia, Kislev, CD Big Hats, Khemri, 40K all metal guard ranges, … if you judge from eBay prices, all those seem pretty lucrative but probably much less so if you factor in the expenses of doing a one-off production run.
And if it comforts you, we also never got a reply for this petition beyond the standard customer service texts.