@Redneckerz: ZDRay is still under development. Nothing is set in stone there yet - hence why I'm discussing with Nash, Boris and Graf where the best place is to put this data.
I can give you basic feedback of the current plan.
The TEXTMAP lump contain two new actor/thing types: static lights and light probes. ZDRay is a ZDBSP replacement. It performs exactly the duties as ZDBSP, except it also generates an additional LIGHTMAP lump. That lump contains the lightmap texture, the texture coordinate info that maps subsector, sides and 3d floors to locations in the texture, and light probe data for the sprites and 3d models.
The general idea is that UDB will get editing support for placing light probes and static lights. Then instead of calling zdbsp it will call zdray and the resulting wad will be ready to be consumed by GZDoom. That's partly why the documentation of zdray itself isn't particular important - it will just be an overview of the new arguments that weren't there already from zdbsp. This is all behind the scenes plumbing for the mappers. For them this is just a few new tools in UDB and the rest should be as they've always known doom mapping.
@Graf: My original idea was actually to read this new information from an input file, possibly the mapinfo file. I'm not sure if it is Nash or Boris that came up with the idea to put it into the TEXTMAP lump, but for me it basically comes down to what is most ideal for UDB. If it already has support for editing the other files in play then we could just put it in one those files instead.