Yes, this is a feature we'd like to implement... but we have yet to find a reliable way to do it.

The problem lies with PostScript limitations: PS1 and PS2 only support transparency for duotone images. PS3 does offer more flexibility, however we have found that not all PS3 implementations (e.g. printer RIPS) support these features reliably. Since we always try to be PS2 compliant (by far the most widely implemented version), we tend to shy away from PS3-specific features, especially if we can't count on them being actually present.

We are considering other options, though (for instance, allowing transparency on raster images when printing through a Windows Printer Driver in PlanetPress Server).

As for duotone transparency color, we'll take a look at what we can do about it.
Technical Product Manager
I don't want to achieve immortality through my work; I want to achieve immortality through not dying - Woody Allen