This is a rather mad idea that came to me this evening.
What if Synfig could export to an .xlsx spreadsheet, and import from an .xlsx similar to one it exported?
You’d have one worksheet per layer type, and attributes as columns within that. Animated attributes would have one column per waypoint following that. (Of course if you had two layers of the same type which animated the same attribute with waypoints of different times, some of the cells in those columns would be empty.)
Like this (it’s a mockup):
The reason would be that many people who lack programming ability do know how to use a spreadsheet. So if they wanted to turn all orange circles purple, or something, it would be easier for them to do it like this than using Synfig Studio, and far far easier for them than modifying the XML.