[PROJ] Synthetic Coordinate System

[Ben Griffin]

I am working on a CRS of the work I’ve been doing involved a series of novel projections. (The current python module is hhg9),
My current  PROJ (C++) implementation shows an RTE (round-trip error) of about ~15nm - and has been extensively tested. 
  
The natural encoding of HEX9 a co-ordinate is hybrid, and involves a separation of the WGS84 ellipsoid into octants (carved at 0,90, etc).
This lends itself to a 3-tuple x,y, octant_id. Therefore I am currently using

PROJCRS["HEX9 Octahedral Barycentric",
    BASEGEOGCRS["WGS 84”, …. ],
    CONVERSION["H9 Octahedral Barycentric",
        METHOD["PROJ h9_boct",
            ID["HEX9","h9_boct"]]],
    CS[Cartesian,3],
        AXIS["hex9 x (X)",east,
            ORDER[1],
            LENGTHUNIT["metre",1]],
        AXIS["hex9 y (Y)",north,
            ORDER[2],
            LENGTHUNIT["metre",1]],
        AXIS["hex9 octant (O)",unspecified,
            ORDER[3],
            SCALEUNIT["unity",1]],
    ID["HEX9","Oct”]]

(This is the registration of the underlying continuous projection — the octant component plays the same role as, for example, a UTM zone number. 
The discrete hex-cell addressing scheme of the supported DGG built on top of this CRS is not what's being registered here).    

The octant component is ordinal/nominal, not a third spatial axis — there's no meaningful interpolation between octant 3 and octant 4. I'm encoding it as a 3rd Cartesian axis with orientation=unspecified and SCALEUNIT["unity",1] because OrdinalCRS isn't composable with ProjectedCRS in current PROJ. Is there a better-supported pattern for this?
Is there any precedent for face-indexed projections (eg, HEALPix has the same issue — 12 faces, each with a local Cartesian frame)?  I considered using EngineeringCRS but that seems to be a mistake - it is strongly tied to WGS84, and lends itself better to DerivedGeographicCRS.

So, the last axis is not strictly Z. 
The X, Y axes are also octant-specific and while X is ‘east’, and Y is on the north/south axis, the direction of north is inverted on some octants.

I will be adding ProjectedCRS to the above base - but I thought it wise to post a line to the list at this point.

Thoughts/Suggestions welcome!