In reply to badgerjockey:
Can those aftermarket products be used on silnylon? Perhaps it "doesn't really last" because it is hard to make chemicals stick to silnylon?
You could try diluting silicone sealant (as in cheap DIY tile gun clear stuff or expensive little McNett tubes from camping shops) with white spirit until you have a half-litre of very thin silicone and then take or make a wide foam brush and apply it to the silnylon? If the flysheet is actually silicone on one side and PU on the other ("taped seams" would appear to suggest it was), like some Terra Nova flysheets, then maybe you could get the aftermarket waterproof treatment to stick to the PU better, in a bath treatment?
You might get a better response posting on one of the (ultra)lightweight backpacking fora, such as trek-lite dot com (uk) or backpackinglight dot com (usa).