We don't know what limitations on the Superchargers present themselves to third-party vehicles since no one has come forward with why they haven't taken the deal.
Tesla has said their are limitations - some of them being software compatibility and others being required to open some of the third party's patents. Which are generally no goes, but understandable requirements.
Some of the issues are pretty clear.
There are a lot of technical & business issues to resolve before that could be possible. There have been several EV startups that have approached Tesla about this.
For one, Tesla SCs would need to be retrofitted physically, or the "foreign" EVs, to let the SCs identify them. Currently, from what I've read, that information (VIN) isn't passed through the car-charger interface, but through the cellular connection from a Tesla to Tesla's servers. I.e., A non-connected Tesla can't currently charge at a SC, IIUC.
AFAIK, no quick-charging protocol allows for transferring information from car to charger beyond the directly charging related stuff like max charge rate & SoC. The non-Tesla DC chargers use RFID or other similar methods to identify the driver/car. I doubt the California law you mentioned will require vehicle IDing .
A potentially usage/business thorny issue is that of precedence: Will Teslas be given priority? And to what extent? If so, will a new Tesla coming into the station preempt an already-waiting non-Tesla? This is particularly an issue because Teslas charge far faster (2X+) than most other EVs, both peak and average rates. Will the session length for a non-Tesla be limited, or maybe even the session terminated if a Tesla comes in?
Personally I think it makes sense for all networks to allow reserving charging timeslots ahead of time (maybe 50% of the available slots,leaving the remainder for "walk-ins") to avoid long waits. I think this will eventually have to happen, once a more significant % of car on the roads are EVs.
If this happens, the issues with 3d-party charging become more complex.
And all the above gets more complex with motorcycles than only charge at 25kW at best.
If I were Zero I would always have on a target board 'what would be required for X system' bikes, so they can check the theoretical price Every year or so as technology changes. Napkin math moves around, and being ready to pounce is part of being agile. You don't want your good thing to lock you from seeking the next thing before it's too late.
From everything I hear about Zero, they still operate far too close to startup mode for that to work; I've worked at or with multiple companies Zero's size or less, and while a many did
intend to do this, it never survived the next company crisis (i.e., a couple of months).