Custom Info Sign: Looking for Options
Of course you can have your own "common" stuffs to place them in all your maps without the need of re-creating them. Apart from copy/paste 
you can use 2 options:
- An Instance: check out this tutorial to know the basics of creating an instance from scratch. Instances are placed into your maps by adding a func_instance entity and make it point to your stored instance.
- A Prefab: once in hammer just select everything that for you make a working group of elements that you want to repeat in every map, then hit the button "Create Prefab" at the bottom of the right part of your hammer editor, then choose a location in your drive to save it.
About all that you described, it's ok, but let me tell you that you only need to pack the custom things, such as custom textures and materials (or models or whatever) but NOT instances or prefabs that you made with the already existing stuff, ok? because all of us already have all of those elements, therefore we will be able to see them all in game.
useful tools and stuff here on TWP 
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If I used Hammer exclusively, I totally would. However, I really like PTI for making the map and Hammer for cleaning it up.
So, I'd like to export my map from PTI but I don't want to make all 50 of the entrance way elements each time. If I could just make it once and then reuse that, it would be great.
I actually have some time now so I'll see if I can make that work...
You should understand that it's a bad idea to export PTI maps unless you know how to optimize. (Lots of 128 brushes textured on all sides without func_detail). Making it from scratch will save you all that. You use instances or prefabs for prefabrications and make your own but that shouldn't be hard to do. Unless I misunderstood.
Thanks guys; you've saved me untold hours of research and guesswork. If I have any problems learning about prefabs, instances, or optimization, I'll make another thread. Looks like the only thing between me and what I want to do right now is homework. 
Good call, I'll avoid releasing unoptimized maps and paking unnecessary things. No more bad mistakes like that from here on in, I promise.
Hey, what does this "vertex tool" do...
EDIT: I've puttered with it and everything works great. I made a prefab with my entrance way complete with sign (thanks to you guys); all I have to do in each map is ungroup the prefab and edit the sign texture. It still keeps all the lovely triggers and stuff that I liked from the PTI entrance way. All I have to do now is decide what I have more patience for: optimizing a PTI map or learning how to do every little thing in Hammer.