Creating premultiplied alpha channels with png and other formats

Hello,

I'm in a process of making assets for Unity3D engine and I'm making transparent png's with alpha channels premultiplied with black. My questions:

1. To this day I've been using superPNG plugin to add alpha channel because it was really hard to find option to do it in Save As... menu. Today very very accidentally I've found Export -> Render Video where I can choose PNG and premultiplication. Am I doing something wrong or is it the proper Photoshop way to export png via video function? It seems really hidden!
2. I can do the same for TIFF, PICT(i suppose no pict for me because its pure mac extension?), and tga format with one black layer and ticking Alpha Channels box, but its somehow greyed out for PNG? Why is that?

I think its some kind of inconsistency, or am I missing something?

Thank You for your time,

Peter