Keyframes in LRT are given 4 star rating so they can be viewed in LR. However sometimes while extensively editing in LR I like to shift keyframes or assign new keyframes. I know this is not recommended procedure, but it would help to have the possibility.

I thought I could give new keyframes a 4 star rating in LR and then save metadata and read in LRT, where they would show up as new keyframes. This doesn't work so there must be some additional metadata used by LRT?

That should work. But if you have the filter to LRT4 Keyframes set, then you will only have filtered for the recently changed keyframes and if you then save metadata, you won't save the metadata for the images where you removed the keyframes from, so those will be preserved.
Make sure to save metadata for all images (Filter to "LRT Full Sequence") after changing stars in Lightroom.