Cottonwood
Roadster#433, Model S#S37
I don't have any direct experience with the HPWC. Obviously modifying it will void its warranty. I think that at the end of the day it may be a lot easier to simply construct the EVSE variant of the Hydra with two J1772 cables. With 6 AWG input wire with 90 degree insulation, and with 75A contactors, you'll be able to do one car at 75A or two at 37.5A. In principle, it would be almost exactly the same thing I did with my Mega Hydra, but taken one "notch" higher.
I went with 50A simply because I wanted to not have to hard-wire it (as an experimenter I am always plugging and unplugging), and because I presently only have a 30A car anyway (and only a 40A circuit in any event). Also, the 50A contactor I sourced was particularly inexpensive.
The only tricky part is that you have to kind of squish 6 AWG wire to get it to fit through the GFI coil. But it can be done (it was done by an OpenEVSE builder once upon a time).
The "splitter" variant board requires a J1772 pilot and proximity line on the inlet, and requires J1772 pilot signaling on the output. If the HPWC can be coerced into working that way, then in principle you could "insert" the Hydra inline and break out an additional J1772 plug. But for the cost of an additional J1772 cable, you get to keep your unmodified HPWC as a backup. I think that'd be valuable.
Thanks for the answers. Here are a few of my thoughts.
As far as I know, both the Tesla Roadster and the Tesla Model S are electrically J1772, but have their own physical connectors. That is why the Roadster and Model S J1772 adapters are just "plug adapters" changing only the physical shape of the connector. Here is info on changing the output of a UMC to a J1772: Tesla Model S UMC cut open and modified to J1772, and here is where you can order a conversion: JESLA is THE 40 amp J1772 portable charging solution!
Clipper Creek sells a 75 Amp J1772 cable to convert a Roadster UMC to a 70 Amp J1772 EVSE. Although I don't know of any examples that have put a J1772 plug on an HPWC, it should be possible.
My reason for this is to get 70 Amp charging for my Roadster from my HPWC. Tesla makes a Roadster Connector to Model S adapter, but they do not make a Model S to Roadster adapter. Because I have a J1772 cable for my Roadster, if I can have a J1772 output from my 80 Amp HPWC, I can use that with the J1772 Roadster cable to charge the Roadster at 70 Amps. On the other hand, I don't want to give up the Model S connector on the HPWC, because it is so convenient.
The conversion that I am talking about is non destructive, because I will not be changing the output cable of the HPWC, only disconnecting it and connecting it to the output of the Hydra.
After looking at all of this, I think that, although very nice, the Hydra is overly complicated for my application where I only need to charge the Model S or a J1772 car (my Roadster), but not both at the same time. I think that what I will do is connect the AC power and ground to the two cables as a simple "Y" connection and put in a toggle switch for the J1772 signaling line so that the active signal only goes to one connector at a time and the other will get a ground or an open, whichever seems to work better. I guess I could also use that toggle switch to control two 100 Amp contactors so that only one output was active at once, but because this is just for my use, that may be more complication than I need. See Lloyd's TS-70 to J1772 conversion for Lloyd's conversion of a Roadster HPC to Roadster and J1772 in parallel. I am just talking about doing this outside of the HPWC in a separate box. There is no hope of putting this all in an HPWC; it's very tight inside.