Right, so I promised to post something about using flowlines:
So, here's the step-by-step guide:
1. Make some points along the seam between two continents in the process of breaking apart
2. Create Feature and choose "Flowline" from the list
3. Next up, choose the appropriate settings. There are three options: spreading centres, left-plate end-points and right-plate end-points
Now, a word of warning: choosing the wrong option here will cause GPlates to crash 100% of the time when you attempt to create the feature, and to make matters worse, I don't know if there's any way to know which option is the one that works in any given situation. I've found the spreading centres option to work most of the time, but occasionally left-plate end-points has worked better. In practice, they seem to work in largely the same way, so I'm not entirely sure what the intention behind the different options is (possibly this feature is still a bit WIP, since GPlates gets updates every now and then).
What this means is that you should always save everything before attempting to create a flowline, just in case it does crash.
Ok, moving on...
4. Next, insert an array of time instants
This will have an effect on at which date the "arrows" along the flowline appear. Should become fairly self-explanatory after reading the next step.
5. Save the feature in a feature collection (probably a good idea to have a separate one for flowlines) and view the results
Finally, assuming everything worked and GPlates didn't crash, you should have a nice flowline appear. Now, keep in mind that since you placed the original points along the seam where the continents drifted apart, this might not 100% match the actual mid-oceanic ridge, because all those strike-slip faults give a bit of wiggle room to oceanic ridges (the shape of the Atlantic Ridge doesn't 100% match the shape of the South American and African coastlines as a result, to give an example).
However, since new oceanic crust is created at equal speed on both sides of the ridge, those yellow dots should accurately depict the position of the ridge, and give a fairly good idea about the shape of the ridge as well.
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