a trash bag inside is good, but they generally don't last real long. Of course you can carry spares at what - 3 ounces each? I like to use a pack cover on a winter blizard trip where the pack will be inside the tent later and keeping the outside of it dry is important, and it does provide quite a bit of protection to the contents if its a well made cover that fits your bag and has drain holes in the bottom (and you don't put it on upside down) .

considering the extremes that manufacturers go to over an ounce, probably lightweight packs are not gonna be watertight if it takes an extra half ounce of seam tape or anything.

Seam sealing your gear (pack) with a thick bodied plastic glue like "seam grip" will reinforce your seams and make them water tight. Even my WM goretex sleeping bag and my Bibler tent are "seam gripped". Note: - use the accelerator and mix small batches, apply with a brush or toothpick. Yes it may add an ounce or even more, but I can sleep in a blizzard in my sleeping bag with no tent or bivy sack.

I guess you're probably better off with a completely watertight durable liner bag and ziplocks. How much weight does that add to those tiny UL packs? a pound?

Soap box on: "It really peaves me that they use too thin of material in tent floors to save weight, then sell you a footprint to protect that floor and don't add the weight to the weight of the tent".
soap box off
Jim