Official comment

Unfortunately, your support rep was half-correct, and not the half you're going to like.
It's not true that 'we aren't fixing this bug because most users aren't using custom objects'.
What's true is that we can't fix this, because it's not a bug. The problem lies with the fact that emails have merge fields and merge fields are related to an Object (contacts, deals, companies, etc), and 'sharing' one email between objects would only serve to break your email.
This isn't a shortcoming of Ontraport, but a fact of life when you are storing different object types. They all have their own campaigns, messages, etc for this reason.
The solution to your problem will be to either rebuild your emails (SimpleMail or HTML emails are easy to copy/paste!) or to move your Contacts to the Contacts object and organize your database differently. Option 2 is a longer conversation, but you can create relationships between Contacts and Contacts that will almost certainly handle your use case.
Most people using Custom Objects are using them for things other than 'people' (Deals, Companies, Events, etc) and keep the people in Contacts.. for the reason you shared and for others (non-Contacts can't buy things, etc).
Sorry this is frustrating! It's not our intention to make your life hard! :(