I haven’t tried, but you should be able to POST many of the variables in now. Give it a go if that’s an option.
Another thing to consider if you’re up for a little coding is to have the click go to your own script, have the script actually create a request via the API with everything set you want and then with the ID that’s returned redirect the user to that new request in HS.
The only thing you wouldn’t be able to control is the public/private note which you’ll need a URL param for in v3 as it required a coding change.
Of course this actually creates a request and I could see a case where you where you want to set a tBody but still leave it modifiable, etc (though you could have them modify on your end). Anyway just an idea that could be more flexible.