There is no reason to prohibit a splice in a grounding electrode conductor. An equipment grounding conductor is allowed to be spliced, and its role is infinitely more important than that of the grounding electrode conductor. Not only is the EGC more important as it relates to electrical safety, it will also carry current in far greater amounts than the GEC, yet has no splicing limitations. Recent revisions in the NEC allow an EGC to act as the required GEC if both wires meet the requirements for both purposes, but the deal breaker is usually the splicing limitation.