According to

http://confluence.atlassian.com/display/JIRACOM/Automating+JIRA+operations+via+wget

one can use a url like

http://jira.atlassian.com/secure/CreateIssueDetails.jspa?pid=10420&issuetype=1&customfield_10010=hooray

to create an issue of a type and project passed as parameters in the URL and also set the value of a custom field along the way, without the form being submitted yet.

This used to work in 3.12, but now it submits the form...