Yes there are two ways I'm aware of, and there may be more than what I'm mentioning here, as I'm not always 100% aware of the features and power of dApps...please bear that in mind.
1. Set your form's up on your website to request a custom installer for a client (client submits the form, you get an email with their info), which a few times a month you schedule time to create custom installers and email them out (with your preferred delivery system, dropbox, skydrive, your website, etc.).
2. At the end of every month, schedule your current Freemium Installer (which you upload to your site) to switch to blocking all of the built-in services, and displaying your webpage (an ad, for the premium version). Then create a new Freemium Installer/re-compile, and upload back to your site (this may require the use of 2 client id's, I'm not sure currently) and restart the cycle of use.
Be sure to monitor who has previously used a Free 30 day trial of your product, using your Forms, and prevent the same users from signing up again and again. I honestly would go with option 2, because it represents the least amount of labor/time involved in maintaining it.
If you require help getting anything setup just PM me, and I'll see what I can do. Good Luck!