Go to a bike shop with the proper tools, borrow a 1/8 chain whip, or wrap a scrap piece of 1/8 chain around the cog and put it in a vice, twist with care. I guess the last option depends on how nice your hub is! It does work, my friend does it all the time, never had a problem.

I used a 3/32 chain whip for several years before buying an 1/8 one. Have you tried it with an 3/32 yet? It just may give you the grip you need. I only bought the 1/8 one because the 3/32 whip tore off the handle. I knew it wasn't the right size, but it worked for me a least 5 or 6 times (tight or loose)

Rotofix idea sounds good. I looked at the link and can't say I was smart enough to get it but I guess it would make more sense with a bike in hand....