Not trying to douse your BP20 fire here, but thought I'd comment on the mix of A and B units, having the ability to purchase individual units to make up whatever combination the customer wants...
I had this sort of discussion with Scott as plans were being formulated for the recent EA/E1 B&O/ATSF production. B&O's EA's were ordered as AB pairs from EMC. But Santa Fe's order was not so. Of the 8 numbered units of the 2 Class of E1's, only numbers 2-4 were ordered as AB sets. Engines 5-9 were ordered/received as A units, only. Single units were deemed sufficient for the early Warbonnet streamliners that originated as 5-9 car consists, and plied a relatively flat route. The LA to San Diego San Diegan was one such consist. A single A unit bearing Leland Knickerbocker's iconic paint scheme was sufficient. And I would have dearly loved having a singleton A unit, only, to replicate this train in addition to the AB set I had already ordered. Actually, availability of a singleton A unit...IMHO...would have been of interest to other buyers, especially those having less interest in the Santa Fe flag, but appreciating the style and paint scheme for its historical mark...but not wanting to pay the price for the B unit. I have no idea how many other Warbonnet nerds like myself might have made the same appeal to Scott. Heck, maybe I was the only PITA on that issue!!!
However, it would seem that the nature of this sort of contractual build dictated that all units, EA/E1, B&O/ATSF, would be as AB sets...ONLY. Never had any explanation, and frankly I shouldn't have expected any. It's Scott's business. If it complicates having these engines built in limited quantities at an affordable price, let it be.
So, extrapolating to this BP20 discussion...mixes of paint, lettering, road names, A's, B's, AB's, ABA's, etc., etc....I'd say 'How badly do you want the BP20's?' I'm thrilled to have my EA and E1 AB sets. If I live long enough, maybe there'll be enough interest in a second run someday...and a contractual arrangement can provide ATSF A units, only. (But I surely am not holding my breath!)
Just a thought...
KD