You asked about Tessellation in another thread. I think it can help you with this as it somewhat inflates the mesh. If you are not using Game Base you can hide the body/legs/arms with the shader attached to this post. It is still not a very good solution, but I have not looked into this problem extensively yet.

There is also an asset called Mount Points that is specifically for this kind of swapping of wearables, but again I have not yet tested it.

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CC3 & Daz Tricks | CC3 to Unity workflow

I initally do a collision detect then expand clothing 1.5 - 2 then I use a verity of poses sitting, flexing. and expand clothing around clipping areas, Gluts / Elbow / Thigh / chest and are bend, / knee. 
I know some just include body with the clothing leaving the main body just a head so changing clothing sets is really just swapping out bodies. (with the skin hidden per clothing set.) 
I have yet to try the alpha on the skin as its about as a hassle as hiding hidden skin. 

Thank you for sharing.
I am doing a similar workflow, but only using C3. By hiding the mesh of the character, and "delete hidden mesh" on export, C3 allows export of clothing only.

One question for you though, how do you avoid the clothes clipping int othe body upon animation? Even with default weight painting on the clothing, it happens in some poses.
I've thought about doing masking on the character body mesh (e.g. making the skin invisible), but don't love this idea.
Manual weight painting seems very tedius and not even sure it will work.

Thank you for your suggestions!

Great to know an option exists.  This is likely very time consuming for any large number of base model and armor combinations.  Has anyone come across an easier approach, or is this pretty much it?  It would be great if CC3 could export pieces as described above.

Theres alot of methods. Persona / Tales of Berseria have outfits where armor only changes with the weapon or sub weapons, Some game have all modular parts where the mid arms and legs are seperate meshes that merge together in multiple parts. Look up modular character creation. Theif had the same head with different bodies for NPCs look up GDC breakdowns / post mortems they shoiuld have several coverages of different techs. 

Thanks, that's an interesting solution. I wonder what others are using.

From studying FF11 and FF13 they seem to still have the character mesh fully visible with their clothing when playing the game and clipping into their clothing. The cloths are made to hover away from the body to avoid poke thoughs that way. Keeping that in mind in Unreal I setup my characters like this: 

Setup for all modular character: 
1) Export Character with an armor set
2) In 3ds max / blender/ maya etc Load the actor with armor. 
3) Select skeleton and actors body / hair - Export selected as Base Mesh
4) Export Armor Set as a whole or as parts using export selected. 
5) You should have a character base that is nude and armor parts that are rigged with no body. 
6) In UE4 I load the character base mesh. And then load on a seperate skeleton object the armor layer and bind the animation pose of the armor actor to the main base mesh. So the animation BP drives the 2 skeletons in unison. 

Any time i switch armor I just load the mesh data for that part onto the skeleton that is marked for that armor / weapon type. 

Suppose you have a traditional RPG where a character can wear and hold several items out of 100's or more. Obviously those (potential) items can't be part of the model when it's imported into Unity, instead they have to be dynamically loaded and "attached" to the rigging as needed.
What is the workflow from CC to Unity for this?
Thanks.