I saw in the November 2018 update, that it added support for custom controls for BPFs in public preview. But I can't see where to turn on the preview or when it might leave preview. I could really use it for my BPFs right now and would love to know how to turn it on.
I can see that the functionality is supposed to be included in version 9.1 18122 of Common Data Service for Apps, which is released in most geographies. However, I can't find anywhere to change the control of the field on the BPF. I would imagine it would be on the Process Designer.
I also tried on the BPF entity itself. But I can't find anything there.
Hopefully, Microsoft will update the documentation soon.