Yes, there is a way, not very nice, but this has been quickly added for a user who wanted this too. You can use the "Registration code" screen for that :
1. Click the EDIT button above the tabs to open the Versions dialog box. Enter CUSTOMTEXT as product code and make sure the Short Codes option is NOT selected. Do it for all the versions for which you need this option and then close the dialog box.
=> the installer will display a fake registration screen, you just have to modify the content of this screen, see below.
2. Select the Wizard Texts tab, select the "Registration (long codes)" page and click the EDIT button on the right to open the dialog editor. Double click the Browse button, uncheck Visible and check Disabled, so that it doesn't appear. And then close the dialog editor and change the texts of the Registration page as you want (you can also resize and move the controls in the dialog editor).
3. To retrieve the text entered by the user in the registration screen you can use the #UserRegCode# macro. For example, if you want to add this to a config.ini file, select the Install Info tab, click the first "Add" button, and enter :
config.ini
your_section
your_entry
#UserRegCode#
=> the text will be written to the your_entry value in the your_section section of the config.ini file.
Hopefully this still works in the latest version, I don't remember when this "feature" was added.