Yeah that's probably the way of doing it, id wanted to stay away from creating friction on the line, thinking that wear might be an issue, it would be nice if the braking force was proportional to the speed too, so you get a smooth start/stop on it.
I've got a load of 608 bearings kicking around, so maybe i could do something like you suggested, but pinching the line between two bearing races, rather than static surfaces....
You aren't looking at a lot of drag, I very much doubt that you will see noticeable wear to the cord as it is moving past, you will see a small groove develop in the plastic but once the groove has worn you will have a fairly big surface and low loads with two very wear resistant materials. If the cord wears out after 1000 uses then I doubt it will be a big deal and your final item will be a lot smaller and easier to transport and adjust.
You will get a smooth start naturally anyway, and if your speed isnt too great and the line has a mellow-ish gradient and isnt too tight then it will create a dip in the line that will probably create some deceleration at the end too.
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G.