Fake transparent clothing layers

*DISCLAIMER*
The sole purpose of these clothing layers is for use inside #RLV shared folders, along with either a RLV relay with the "Smart Strip" feature or with a viewer handling #RLV composite clothing (which unfortunately do not exist anymore in the mainstream :().

They are meant to be worn alongside with clothing attachments. They are invisible and will not change the appearance of your avatar.
Their role is for mesh and prim clothing to be handled by RLV devices in an almost transparent way as if they were legacy clothing layers, i.e.: they will be listed as such in the various menus of RLV devices, and sending the command to strip a legacy layer, will actually make all relevant prims, meshes and alpha layers go away with it.

*Smart strip*
Smart Strip is a feature of some RLV relays (to my knowledge only in Dahlia's, Satomi's and its forks) that deals with "composite" clothing items (composite in the sense they are composed of several inventory item, for instance a clothing layer plus a few prims, and typically nowadays, a mesh attachment plus an alpha layer). Smart strip intercepts all RLV commands related to clothing layers (i.e., all commands having "outfit" in their names) and makes them act also on the whole folder containing the concerned layer. For instance, stripping "pants" will make the avatar remove the pants layer AND all the folder that was containing the pants layer, typically including belt and cuffs prims.

Hence Smart Strip works well as long as each composite clothing folder contains at least one clothing layer item. Unfortunately, full prim clothing (including the now popular mesh clothing) usually don't need to use legacy clothing layers and therefore include none. Here come these "fake clothing layers" into play.

*How to set up these fake clothing layers*
To set up this system, you need to use #RLV folders and make a subfolder for each "composite" clothing item (typically, a full outfit will contain several such subfolders, one for each piece of clothing), the same way you used to do for using Smart Strip if you were already familiar. When it is done, find folders that do not include any meaningful clothing layer and add one (or more!) of the "fake" ones to each folder. Which to choose depends on what the composite clothing item is supposed to be. If it looks like pants, just add a copy of (or link to) "fake transparent pants". If it looks like a jacket or a coat, add "fake transparent jacket", and so on.
When you are ready, enable "Smart Strip" in your RLV relay.

*Link or copy?*
In concerned folders, it is possible to either add either a link or a copy. Both ways will work, albeit a little differently.

Avatar rebakes (advantage for links): Working with links has the advantage you can avoid avatar texture rebakes when replacing a clothing composite by a composite of the same type (for instance pants replaced by pants), as the clothing system considers the avatar is wearing the same textures. If you are working with copies, even if textures on the avatar mesh won't change, replacing a clothing layer will always trigger a rebake.

Accurate information on worn folders (advantage for copies): if you wear fake transparent pants and those pants are linked in several subfolders of #RLV, then @getinvworn will report all folders containing a link as partially worn, when they are not supposed to be worn. You don't have this problem if they were containing their own separate copy.

*So it's only about relays. What about direct RLV commands from attachments?*
For direct RLV commands, if you have the permissions to modify your attachment and then it is possible to script a feature equivalent to "Smart Strip" (I assume most seasoned RLV scripters will immediately see how it is possible). The other possibility is to have a viewerside implementation, of which I know only one: Kitty Barnet's "composites" that used to be in earlier versions of RLVa. The C++ code is still in her sources, although disabled since RLVa viewers have been handling inventory links. Just re-enabling it won't directly work (there have been too many changes).
Fortunately, since that time, I've been maintaining my own viewer branch (based first on Phoenix, now on Firestorm) with working composites. If you are a viewer developer and want it in your viewer, I could easily derive a patch that would work with your viewer, provided its RLV implementation is RLVa.