Hello Rick
This may come too late but I noticed that you did not get much of response to your posting.
The German-made Pro35 Adapter does work on this camera and I have used it with Zeiss Ultra Primes.
P&Shttps://www.pstechnik.de/en/digitalfilm-pro35.php
It’s definitely a RENTAL item as the purchase price is close to that of the HDX900 body — Abel rents it to you (LA/ NY) so do a few other vendors.
https://www.abelcine.com/store/home.php?cat=615
I would be careful about the look though and do a test shoot first — you definitely get the desired shallow depth of filed but at a price:
1) considerable loss of sharpness
2) loss of speed (2 stops) so no good for low light situations
3) loss of contrast — you will get a slightly dreamy/smeary picture – which may or may not be the look you are going for. The effect is similar of what you get when you shoot with a WHITE PROMIST filter.
Also be sure to white shade using the adapter (to avoid green / purple shifts)
Another possibility you ought to consider is to go with DigiPrimes (Zeiss) or Canon Cine Style Primes. You maintain sharpness while being able to shoot wide open and get a shallow depth of field (not quite as dramatic as with Pro35) w.o giving up resolution, contrast and speed.
Hope that helps — hit me up if you have any other Q’s
High-Definator