Generating user content in Jenkins
I have found the way to do this:

install Scriptler plugin
write Groovy script that implements some additional functionality needed by Jenkins users
write webpage that uses Javascript + jQuery to use form elements' values for GET/POST to Groovy script, update the webpage dynamically (say by replacing html body or adding to it), put it in userContent
grant selected Jenkins users Run script permission in the Jenkins' security matrix config
For non-technical reasons I need to keep generating user content in Jenkins.
Theoretically I could do smth like:

have parameterized build
provide webpage in user content folder that does GET/POST to parameterized build
display webpage with results (I don't even know if it's possible)

UPDATE: That is, I want to run some dynamic webpage in Jenkins (yes I know it does not look very good). Specifically, Jenkins users after logging in need some additional functionality like generating paths and hashes from job workspaces and have them displayed and running such logic as a separate Jenkins job is not very attractive (user content folder is simply the most appropriate place for such stuff I think). Typically, I'd provide such features using say simple Django webpage, but that's not an option for various reasons.