How to mask a layer with alpha from another layer that is NOT above it?

Yeah, as Gutterfish said, simply duplicate the layer. Pre-composing such layers so the resulting comps can be reused over and over again is generally a good idea and would also allow to use effects liek Set Matte, Channel Combiner and so on to transfer the transparency info to otehr layers without more duplicates required.

Mylenium

No need to overthink it.  Just duplicate the layer and move the duplicate directly above the layer you want to use it on.