I want to add some static fields to my Lua userdata objects coming from C++. The objects in question are vectors, they're created in C++ land and they work as they are but I've tried lua_setfield on my userdata but I get a attempt to index a Vector value error and I don't really want to have to use .x(), .y(), .z() due to the cost of having to call a function, push to stack and then read on the Lua side.
Is there any way to register fields on userdata for Lua access?
Userdata doesn't have "fields". What it can have is a metatable, for which you can define the __index and __newindex metamethods. Given such a metatable, the former function is called when reading the value of a field of the userdata, while the latter is called when assigning a value to a field from the outside (ud.some_field = 4 or an equivalent). If you want fields to be read only (to some degree), you can just implement __index, and attempts by the user to use __newindex will fail.
So in your C++ code, after creating the userdata, you can use lua_setmetatable to assign it a table that has these methods defined within it. Of course, these functions will need to access the actual C++ object from the userdata and fetch the specific values you're interested in.
