The problem with this is that Unity doesn't seem to purge memory between building assets in Resources. All the platform collections go into resources, so at some point 32 bit unity is going to run out of memory building textures in resources. This isn't tk2d specific, its purely down to that specific issue above.
As for workarounds... I can think of a couple
1. Dont use platform collections, but rather turn on mipmaps, and QualitySettings.masterTextureLimit
2. Modify the tk2d code to allow loading from asset bundles. This will involve a fair bit of work.