Hi,
Well, I understand what you mean, but from experience, I don't see many uses for that.
Here for example, doing what you want requires at least getting information on the current product, running a MySQL query and some processing on the results of the query to be able to display the percentage. So yo'ure looking at at least a dozen lines of code.
If you know how to do that in Joomla, you likely already know how to put up a Joomla plugin (a xml with 10 lines, and a PHP file with a few lines). So as a developer you should be able to easily put up a plugin together to do that and it wouldn't take you much longer than just writing your PHP code in a PHP custom field.
What's more, usually, when you want to customize custom fields, you want to extend from an existing custom field type. That way, it simplifies the code you need a lot.
Having a "goal" option in the donation plugin is a good idea. I'll note that on our todo list.
Regarding supporting Joomla custom fields, I've looked at it in the past, and it's not practical. HikaShop doesn't use Joomla's model system because it didn't exist when HikaShop was created. Also, the structure of the custom fields in Joomla is quite different from what we have in HikaShop. We would basically have to rewrite a big part of how HikaShop works. And the gain are not that important since most of the things you can do with the Joomla custom field types can also be do with HikaShop's custom field types.