If I could only do one thingfor my grip it would be long distance bear crawls on a grass field. Doing them myself, I have noticed considerable overall improvement in my lower arms. Essentially it's because my hands are acting as my feet with the bear crawl. Pretty easy, no equipment necessary but it is a definite conditioner. Not for everybody.
For your side question on hand balancing...I have a strong feeling that bear crawls would carry over to hand balancing. The connecting factor is that both the bear crawl and the hand balance require the hands to act as weight bearers. I figure the stronger your body becomes weight bearing, the more improvement you can make hand balancing.
One last thing...I think hand balancing is incredible training but anyone who goes for it should definitely have a prerequisite level of upper body/core conditioning. There's a progression for everything. Hope this helps you.