Hi,
You may be able to use the 'Edit File Associations...' feature in the 'TextMate' menu
I just tried it with php files. I made the 'File extensions' textbox blank, and inserted '<?php' into the 'First line pattern' textbox. I made the language 'PHP'
It works. I guess this will only work for you if there's a common pattern on the first line of these documents that you can match.
I also found that the 'First line pattern' is ignored if there's anything in 'File extensions'
Hope that helps, Ian