Unable to add multiple participant registrations for one contact
As I understand you correctly, you were exploiting a bug, and now its is fixed you search for another solution. One option is to undo the bugfix.

You can do this with the following actions:


Lookup the issue (CRM-19550).
Find the pull that contains the patch (https://github.com/civicrm/civicrm-core/pull/9303).
Create a patch file


wget https://github.com/civicrm/civicrm-core/pull/9303.patch > CRM19550.patch


Reverse apply the patch on your 4.7.14 installation


git apply -R CRM19550.patch in the [civicrm.root].

You have to follow the same procedure with every upgrade.