If it is a GETITEM template tag you want to use, then you don't have to use it in the specific template that is tied to your documenttype.
For example, this would be fine (pseudo markup):
Code:
<mastertemplate>
<head><GETITEM field="myTitleProperty" /></head>
<body>
<child template>
<GETITEM field="myTextProperty" />
</child template>
</body>
</mastertemplate>
Umbraco will assemble the templates to _one_ template, that will then be rendered with the current pages data.
Does this help? Or is this not what you meant?