stdin mode, length limit, a lot gets rejected

[epixoip]

1. There's nothing you can do about it, the length is simply too long for any optimized cracker. You'll need to use a cracker that just uses OpenSSL or something.

2. oclHashcat is not the correct program to use for this anyway. Since you have no amplifier, you are gaining no acceleration. This attack will be faster on CPU than GPU regardless.