If you want to use meeting invites, it needs to be a meeting, not a custom entity.
You could write code to send an .ics file for the meeting for example, but the point of an Outlook meeting invite is that the organiser can track who has said they are or are not attending, or have not yet replied, for example.
Note that more recent versions of CRM *do* send invitations out for appointments created in CRM, depending on how you configure your synchonisation (eg using SSS they are automatically sent out).
Hope this helps.
Adam Vero, Microsoft Certified Trainer | Microsoft Community Contributor 2011
UK CRM Guru Blog