ABS_GRAPHICS,
Transparent text(and transparency in general) is not supported by PlanetPress. The only supported transparency is duotoning with monochrome black and white bitmaps(they must be saved as actual black and white bitmaps, a 24 bits bitmap that only uses black and white will not work), so it's extremely limited.
If you use an image that already has transparency in it, such as a png file, sometimes the transparency will carry over when previewing or printing. However, this is not officially supported and there is no guarantees that it will work.
For transparent text, I don't see any ways to do it in PlanetPress.
Regards,
Rapha