DBs will be easier to control than long farmers, but they're going to want to "roll" out of your grip, which farmers wont do. I've taken 300lb farmers for 50' I can't imagine I coudl do that with 300lb DBs
I dont think the EZ curl bar will roll tho, you might be onto somehting there. Try checking out the Diesel Crew's site, or ask Smitty on EliteFTS, I seem to remember them doing something with a row attachment for a cable, using that as a handle and hooking it onto EZ curl bars.
Regarding the original question - COCs:deadlift/farmers grip strength as pushups:bench press
If you've never really benched or done pushups before than one can help the other. But at a certain point they are more independent of one another. Similarly if you can't close the trainer, then working on crush may help you, MAYBE it'll help you if you cant close the 1. but beyond that they seem different. Many people are fans of static holds with the gripper tho, albeit no 1st hand experience on my part.