I can not make a drone place a sign!
I'm going to share this with the developers! Let's see what's going on...
I have the feeling you may be using the 'sign' material, which is the entity/item. You want to use sign_post, as I have done in this small test mod: http://mod.learntomod.com/programs/balthatrix-test_sign
Does that help?
How to add/change text to a sign_post in learn to mod?
You could try using the texture editor on learntomod. Let me know if that doesn't work.