Chain is a bad idea. Besides safety considerations, chain will go slack when the front of the tractor is higher than the mower. Slack chain means the mower tips forward, which means the revolving knives get closer to the ground. Depending upon the degree of angular difference, that can temporarily turn the flail from mower to tiller. The opposite is also undesireable, say when cresting a hill. The nose of the tractor goes down, the chain pulls the mower up; often to the point of no cut at all.
A chain will work however, if you mow parallel to the dips and rises - as opposed to up and down them. That usually leaves a narrow strip at the bottom of each valley, but is much preferable to scalping and missing on each dip and rise. But this procedure can be difficult with narrow track tractors that tend to be a little "tippy". In that case stick with the rigid toplink, and mow up and down as you originally stated.
//greg//