Form Validation: Custom Error Messages based on Field?

[eluser]wolfman2000[/eluser]
I do not quite know if this is possible here. In the Form Validation section, it takes about setting custom error messages for both the standard options and callbacks. While that is great, I want something a little different.

One of my form fields has 3 checks on it: overkill perhaps, but work with me. Two are standard checks, and one is a callback. I want it so that if ANY of those checks fail, I have a custom error message to just that field. I don't wish to have to overrule any of the default messages.

If this is not possible...is there a way to call functions such as field_required or field_is_natural_no_zero inside a callback? That could potentially get me what I'm after as well.