I tried escaping, but that delivered similar results. After trial-and-erroring some variants, it looks like I’ve found a way to do this: using @@ and removing all single and double quotes in the command variable. The only quotes remaining are those in the body.
This code seems to work, I will monitor in the coming days and report back.
var String token = "mytoken"
var String api = "https://www.mindergas.nl/api/gas_meter_readings"
var String pubdate = "2016-05-19"
var String pubstand = "988.545"
var String body = '{"date":"' + pubdate + '","reading":"' + pubstand + '"}'
var String command = 'curl@@-s@@-w@@%{http_code}@@-k@@-H@@Content-Type:application/json@@-H@@Accept:application/json@@-H@@AUTH-TOKEN:' + token + "@@-d@@" + body + "@@" + api
var String result = executeCommandLine(command, 5000)
@rlkoshak Thanks for your help.