WPB Material and Lining?

There are usually two reasons for using a lining on a WPB fabric:

i) to protect the coating from abrasion (if it's a 2-layer fabric);
ii) to stop you noticing condensation.

For a bivvy, I can't really see that either is applicable; you don't get the same degree of continuous movement that you do with clothing (so wear shouldn't be a big issue), and any condensation (of sweat) is likely to get to your sleeping bag through a liner, anyway… Since you're not very active when in a bivvy, condensation is likely to be less of an issue than in clothing (assuming you're venting your breath somehow).

You could email them about ordering a sample, and see how the coating stands up to abrasion.

That is about 1.5 oz/sq/yd. Lighter than some silnylons. Too bad they only have it in black. But note that some of the vendors give the weight of the material before the coating is added. You could inquire about that also.