That's useful, but doesn't negate the original concern at all.
You can't easily form a dynamic json string for submitting to /build by accepting user input from form fields. It's a usable workaround when everything you want is known at job-completion time. But sometimes you want to prefill most parameters, but allow some to be adjusted. That doesn't seem to be possible with your approach unless you're able to enable JavaScript and build some json on the fly from the form input fields.
Allowing buildWithParameters to redirect to the job afterwards seems like a low-cost solution, perhaps with a redirectToJob=1 param you can make a hidden input. So you can still have your nice custom form, prefilled with most of the params you want to forward to the new job, and you can let the user modify some params before submit.
AFAICS to work around this you need JavaScript to submit without navigating to the empty result page, then to redirect to the new job after a delay.
2 things to mention here :-
1) I'm sure the /buildWithParameters used to work and successfully redirected to the job page, so useful behaviour was lost (I get asked for it back every week or so)
2) The patch in the linked issue seems to try and include a redirect, so is it trying to do what this issue asks for and failing, or am I reading it wrong?