Can you roast vegetables before freezing instead of blanching?
Yes you can, however they will suffer the same problems with texture as you find with the blanched beans. This is because it is the cooking part that is affecting the texture, not the freezing.

You can also freeze green-beans fresh without blanching. All you need to do is rinse, slice (if you like), then freeze. They may change color when cooked after thawing to a more brownish green, but the texture is less mushy in my opinion.