I have an ale that will have an OG of .073, but only 1 smack pack on hand and brewing in 12 hours. Nervous that the 1 pack may not be enough to support the OG? I always have an extra few dry packs of SafeAle 05 on hand. Wondering if just rehydrating one and pitching in addition to the smack pack is advisable, or just take my chances with the one smack pack?
I would rehydrate the US05 and pitch in with a good healthy cell count. You will not get the full yeast profile of your intended yeast strain, but you will avoid getting off flavours produced by stressed yeast, also you mitigate the risk of a stuck fermentation.
US05 is fine to just sprinkle on the top of your brew, you can rehydrate and make up a starter if you wish, but US05 is pretty hardy and can just be dumped in dry.
Just ensure your wort is well aerated and maybe give it a good shake about 12 hours in given the higher gravity of the brew.
I heard of people fixing their underfermented beers with US-05, and generally it was fine, because that strain is rather neutral. I would expect that the character of your beer wouldn't be expressed in full, though.
What I would do - make a starter with that smack pack right now. Add some yeast nutrient there. Oxygenate really well during starter making (I have a stir plate). They say a starter takes 24 hours, but I reckon in 12 hours you're already going to have enough to pitch, especially the original smack pack was healthy.