Copying a gray-scale representation of your image as a layer mask has been possible in Pixelmator for quite some time. Though I agree it is not obvious how to do this at first and requires three-four steps.
1. duplicate the layer you want to use a a layer mask.
2. use the mask to alpha filter on this new layer.
3. use load selection on the new layer.
4. ctr+click on the layer you want to add a layer mask to and choose add layer mask from the menu that appears.
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