I’ve thought on that approaching several times and I found this:
You want a pair of subparameters for the canvas parameter. You have one of them already Time Off Set parameter of the paste canvas layer is a parameter that modifies the time of the paste canvas in relation to the canvas that hold the paste canvas layer.
Say we have a document (head) with a mouth canvas imported from file (or exported internally, it doens’t matter) lying in a paste canvas layer.
If you have a document time of 4s and a Off Set time of -1s you see a time of 4-1=3s in the paste canvas layer. You can make very cool things with the Off Set time parameter. You can convert to a linear variation and make the imported canvas (mouth) go slower, faster or freeze. You can modify it even with constant waypoints to make jumps beetween different position of the timeline in the mouth canvas. See here for example. In the mentioned example the animation is reversed.
But where is the problem? For me the problem is this:
- You store your phoneme poses in the mouth file: AI, E, O, U, FV, L, MPB, WQ, rest.
- Imagine that you store each phoneme keyframe with a separation of 10 frames between them.
- Let’s say you use the Offset and Duration subparameters or the conversion type with the rate and offset. it is similar.
- Imagine that you have the O phoneme keyed at a certain time and you want to make a transition to a ‘rest’ phoneme. Considering the order that the phonemes are stored you cannot make a normal transition form one keyframe to other because there are other keyframes in the middle. So you end going to the ‘rest’ phoneme in a constant jump. So where is the benefit?? there is not smooth transition!
I think that the good approaching is
-Insert phoneme keyframes in the timeline of the exported canvas and make the document time to follow the time of the child canvas. Unfortunately update the timeline of the root document based on the children timeline cause problems. I don’t remember exactly what where but I think that is related to time propagation from child to root and then from root to child in a endless loop
I wish to dig on this one of these days 
-G