Will there be a Brainpool Curve support for ECC signatures in Adobe Reader?

This isn't really about adding a signature handler, but about adding new ECC curves to the current handler. Different incarnations of Acrobat/Reader for Windows/Mac can validate curves other than the FIPS standard P256, P384, P521, but those three are the only ones guaranteed to be supported in all versions. This is a limitation of the crypto libraries used in the product. Adding a new signature handler would mean adding the handler plugin to all copies of Acrobat/Reader that might need to validate Brainpool, which is kind of a non-starter for widespread use (e.g. Brazil and Germany want to use Brainpool for government certificates). You need to find an Acrobat Product Manager and convince him/her that Brainpool support would be a good idea. Good luck.

Are you asking about native support? Acrobat and Reader have a signature plugin architecture that allows third party developers to add signature handlers of any kind to the application. There's nothing stopping anyone from adding this to Acrobat and Reader.