Thank you, Stefan and Esme! (And sorry about the other mail, Esme - I thought gmail would reply to all by default)
Native probably sets it based on EncoderParameters, but I think this is good enough for now.
I used the "Determining the Parameters Supported by an Encoder" example code to query image/png encoder parameters on win10, and if I understand that output correctly, the PNG encoder only supports the "EncoderImageItems" parameter (for image metadata). Or maybe the "EncoderQuality" parameter is supported implicitly without being listed in GetEncoderParameterList? I haven't tried it, but Google found a forum thread where people basically complain that gdiplus can't use prefiltering when encoding PNG files.