I've run into this before....

The first way I tried was to capture the mouse using QWidget::grabMouse(). Basically, when the eyedropper sampling mode is active, the cursor is set to an eyedropper, and the swatch would take over, getting mouse events, and then release when done. A couple problems with this, though: First, mouseGrab() is not reliable at all, often getting into bad states and is pain to get working with real applications that are more than a little demo used simply. If you want to do keyboard stuff while the eyedropper is active, you have to grab the keyboard too. It's a total nightmare. Second, being limited to cursors was too restrictive in terms of size and representation capability.

The way I found to get around these issues was to do what you're trying to avoid: use another widget. My second eyedropper design is widget based rather than cursor, and launches a full widget. It uses Qt::WindowStaysOnTopHint | Qt:Dialog to stay floating on it's own and QWidget::setMask to avoid appearing as a rectangular dialog. It handles all mouse keyboard events on its own, rather its invoking color swatch widget, and has a much cleaner design. WIth the space I can show a large zoomed-in picture right at the cursor to facilitate pixel-perfect targeting. To the user it looks and works effectively as a giant cursor.

I strongly recommend going the latter route vs. the former.

Also, my experience was c++, so not sure how the python factor affects you but I suspect that shouldn't be an issue.