Bear in mind that Phoslock is being deliberately manufactured so that it can readily be turned into a dispersible slurry. But it could be produced in slightly different forms also. I have not seen how the pillow product is constructed and how the product is arranged within it, but it would be expected that the bentonite clay is formable into stable material forms if it is compounded with other materials. The aim would have to be that it is fully permeable to allow water to flow through it easily, while presenting a high surface area to the water, so the PO4 meets the active sites. This can be done by coating high surface area fibres with active product for instance.
What I do not yet understand is how the PO4 can be removed from the chemical complex. In water, PO4 sticks like the proverbial sh!t to a blanket, but it may be quite feasible to remove it using a wash with acid or alkali etc. This would make it like ion-exchange resins which are very widely used for similar extractions. If the product was carefully designed it would be fantastic as an extraction bed for flowing water either in streams and waterways, or in built-for-purpose water treatment plants. The water is stripped of PO4 as it passes through, and can then be back-extracted off-line clean and reused.
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