I decided to test it anyway, so here are the results of a bechamel using 1 part soy to 2 parts regular milk:
Taste:
There was nothing on the label to indicate the soy was sweetened, so I went ahead as if it were unsweetened. However, the sauce was much sweeter than usual, and on checking the ingredients, I note that there is 2% raw sugar (as well as 1% salt). So I guess it was sweetened after all! I added an extra onion and more salt to try to counteract the sweetness, but it was still noticeable to me. My husband said it was fine.
Texture:
In the first stages of adding warm milk (infused with onion, bay leaves and peppercorns) to my butter/flour, I noticed that the roux was a lot stretchier than normal, so I had to add milk more frequently. Otherwise, the overall texture was lovely, very silky.
Thickening:
It seemed to thicken a bit more than usual, but this may have been subjective. It wasn't enough of a difference to affect the use of the sauce in any way. It certainly stuck to the bottom of the pan more than usual :(
Overall:
If I were sure that I were using UNSWEETENED soy, I would try this again if I was again left with soy milk in the fridge. I would reduce the ratio to make 1 part soy to 3 parts regular milk for my personal taste. I added some Emmenthaler and Grana Padano to the sauce, but if using soy milk, I believe the Emmenthaler is too sweet, and should be replaced by a sharper cheese such as Cheddar.
Hope that helps others faced with the same problem.