Thanks to post: Show an activity into the designer? I was able to create an ActivityBuilder with properties:
Properties =
{
new DynamicActivityProperty { Name = "Account", Type = typeof(InArgument<AccountModel>) }
}
And Load() it in a WorkflowDesigner.
This this means that I can now force the AccountModel argument into any user created workflow.
I hope this comes in handy to anyone stuck with the same problem.