[TYPO3] configure BE input form

[Martin Kindler]

I have written an extension which adds my own Ctype(s) to tt_content. 
Now I want to specify the image orientation for the image used in my Ctypes
(which are essentially textpics where text and image come from an external
data base, but with my own rendering). I would like to reuse the imageorient
field from tt_content and also the BE form elements should be (nearly) the
same as the "normal" ones provided by T3. I want to allow only three types
of imageorient (i. e. top-left, top-center, and top-right). Therefore the
other options must be removed from the SELECT. I can do this by modifying
the TCA for "imageorient" but then the options will be removed from ALL
types, which I do not want. 

How do I modify the optione only for my Ctypes?

Martin