Custom fields for certain products


I have been looking for a solution to my problem in your forums but seems like no one has give an answer or the problem is not the same one as mine.

I wonder if there is any plug-in or way (apart from hardcoding it) to do this:
When buying a product, ask for different information (have a textinput field) with the order. But this form should change in depends of the product.

I see that this usually is done during the checkout, but then, how can it be done customize per product? How can I customize the checkout questions?

I know that can be done modifying the mp_checkout.php or mp_cart.php, but this is not maintenable for a non technical responsible person.

Thanks in advance.