Razorback,
With respect to #1, OSHA states 1910.178(a)(4):
Modifications and additions which affect capacity and safe operation shall not be performed by the customer or user without manufacturers prior written approval. Capacity, operation, and maintenance instruction plates, tags, or decals shall be changed accordingly.
The ASTM/ITSDF B56.1-2005 standard reiterates this thought. So, yes you can do it but you'll need the manufacturer's written approval.
With respect to #2, I did not see anything regarding the thickness of the forks but modifying the forks (e.g. 1/2" holes) will change their capacity and thus we're back to to #1. The forks need to be replaced if the damage or wear is sufficient to cause failure. You'll need to get a forklift inspection service to look at the forks and give you a recommendation. The forklift manufacturer or Thomas Register can point you to an inspection service.
Forgive me for speculating but from questions above, it sounds like someone wants to run a bar in between the forks and lift something using a chain or some type of rigging. Don't do it. It's more trouble than its worth. The manufacturer's have al sorts attachements designed to work on forklifts. I'm sure they will have something that fit the bill.
MikeCJ