When I set the OPACITY in the Brush Properties dialog to ZERO by dragging the TRANSPARENCY trackbar to the very left, then this does not work on the FILL TOOL:
But from the user's view, that would be the LOGICAL way to fill an area with Transparency using the Fill Tool.
Unfortunately the color selector is a standard Windows dialog, so we can't customize it to add a transparency option. However in the next release you can hold the Alt key to fill with transparency.
It would be nice if there was an OnPromptForColor event where we could decide whether the standard color-picker dialog is invoked or a custom color-picker dialog. That would be very useful!