As I understand if, the syntax for arrays should be JSON/YAML stype, so if I wish to have an array of hashes the value for a parameter should be something like "[{ "hostname": "host", "password": "secret" }].
Adding this works fine, but when the change is saved, foreman changes the syntax, so the next time I edit the host the value is instead "[{"hostname"=>"host", "password"=>"secret"}]".

Trying to edit anything on this host then gives me the error "(<unknown>): did not find expected ',' or '}' while parsing a flow mapping at line 1 column 2" since foreman doesn't get the syntax it itself created, which means that this value has to be manually changed every time anything is changed on the host in question.