The GivEnergy app is already very flexible and the desk-top portal even moreso. I think they have a demo version if you wanted to have a look around.The API to PowerWall looks OK to me (some exceptions), but so far all that I have found is boy's-own stuff, no polished 3rd party APP.
The only negative I've seen (in powerWall API) so far is inability to tell it to charge up. You can set the reserve to 100%, which will achieve that, but the rate of charge is "modest" which means, for example, that it won't fill up in a 4 hour slot. So still have to set an "Off Peak" slot for the overnight period (which does, then, charge "flat out"), but that's only useful during Winter, and in Autumn / Spring the ability to adjust it dynamically would help a lot ... in Spring I need it to definitely NOT fill right up overnight because although the day does not have enough to charge the battery to 100% (i.e. I need to start the dday with some "grid charge") the sun is up at, say, 7AM and therefore the amount of discharge from end of off peak, to sun up, is not enough to prevent export (if the battery is at 100% at end of Off Peak)
Basically, I want to set a MAX CHARGE "Reserve" based on a prediction for tomorrow's sun and then command "Charge FLAT OUT". Currently I would need to (manually) adjust the timeslot for OFF PEAK rate ...
So once the API allows "Start charging from GRID" (i.e. flat out!) then just setting reserve will be enough ... I reckon!
The app allows you to set a charge level, time slot and also charge rate (watts). You can also set a reserve % in there too. So it's pretty handy to configure. At the moment I'm simply setting the charging time slot to coincide with Octopus Go Faster and then flexing the charge level to match what ullage I think I need the following day. This is pretty easy at the moment as I'm charging to 90% most of the time given that solar is very limited. Come April (ish) I want to be able to automate this based on Solcast predictions and fortunately a lot of this work has already been done and proven.
Some users have configured all manner of permutations and the Giv system allows this as far as I can tell.
Whilst GivEnergy are having to pedal fast to keep up with demand, and to be fair some cracks are showing; their app, portal and general integration capabilites are really very impressive. Worth a look. If they can bring their "Powerwall killer" to market efficiently and at sensible money, then Tesla will need to step up and in no small order. The next 12 - 18 months will be interesting.