Blast.

I've figured out a cheat that works in my situation.

Basically instead of having the transparent rounded corners be part of the taskbar image, I've cut them off and joined them to the images used by the surrounding modules (which have proper transparency) so the taskbar is just a solid bar.

Kind of a shady way of going about it but it works.. I'm not sure if tasks.dll may have better transparency support but switching modules seems like it'd be more work for me so I'm stickin with this ;)