There are tons of custom attributes that we can use with Houdini Engine in Unreal. I want to try and thoroughly document them all in this thread with everybody's help. (If this works out, maybe it would be best to make this a sticky.)
Please contribute! ^^ - I'll update this post whenever I can.

Attributes for export:

unreal_generated_mesh_name
function: Name output meshes
class: detail
type: string (?)
example values: MyHoudiniPrimitives

unreal_lightmap_resolution
function: Set custom lightmap resolution for static meshes
class: detail
type: (?)
example values: (?)

unreal_face_material
function: Assign a material to a face
class: primitve
type: string
example: In the name field, change class to primitive, type to string and enter your unreal materials path (you can copy the link in unreal by right clicking the unreal material and choosing copy reference).

unreal_material
function: Assign a material to a mesh?
class: ?
type: string


unreal_instance
function: create output as instances. For example, make an instance node with a few points in it. Give them a point unreal_instance attribute with the path to mesh object. The whole thing can be used to be baked to Unreal instance Blueprint.
class: detail/point
type: string
example values: Blueprint'/Game/platform_bp.platform_bp'
? ../myMesh
note: - more info in: https://www.sidefx.com/forum/topic/46689/ [sidefx.com]
- unlike instance attribtue allows to specify native UE4 resources that are supposed to be turned into instances

instance
function: create output as instances
class: point ?
type: string
example values: ../myMesh

ogl_numtex
function: ? https://github.com/sideeffects/HoudiniEngineForUnreal/issues/52 [github.com]
class: ?
type: int
notes: also related:
https://github.com/sideeffects/HoudiniEngineForUnreal/issues/51 [github.com]

Groups for export:
Objects in these special groups will be treated differently on export:

collision_geo
function: will be used as collision
notes: - it will be turned into a separate invisible static mesh with per-poly collision
- This is inefficient and should be replaced: https://github.com/sideeffects/HoudiniEngineForUnreal/issues/59 [github.com]

rendered_collision_geo
function: same as collision_geo but the mesh stays visible.


Attribtues that can get imported:
On Landscapes (source: https://github.com/sideeffects/HoudiniEngineForUnreal/issues/7): [github.com]
P
function: Point positions of the landscape
class: point
type: vec3

N
function: Normal, possibly useful for remeshing
class: point
type: vec3

uv
function: Landscape UVs (with uv being 0)
class: ?
type: vec3
notes: - There is an option to import these normalized . By default the UE4 landscape uvs apperntly are not normalized

(?) material names, material hole names
function: ?
class: detail/primitive
type: ?
example values: ?
Global landscape material and hole material as detail attribute.
Individual landscape component material and hole material as primitive attribute.
notes: - Needs to be activated in the landscape input options

name
function: This can be used to identify all points belonging to a certain tile/ landscape component grid/landscape components
class: point
type: (?)
example values: (?)

unreal_vertex_index
function: Can be used to locate a point within a component heightmap grid
class: point
type: ? vec2 ?
example values: 0,0
notes: Vertex indices for each landscape component start at 0,0

unreal_lightmap_color
function: ? Get information about baked lighting?
class: ? point
type: ? vec3
example values: ? 0,0,0
notes: ? Seems to be a color value, maybe

With H16 we got official docs for that:

http://www.sidefx.com/docs/unreal/_attributes.html [sidefx.com]