Hi David. Did you try a lua script to do this? You can draw on a regular basis (with alpha = 255 on RGB channels), then you can run a script to iterate each pixel and convert its alpha channel to 0.
Check Aseprite - Docs - Scripting
API GitHub - aseprite/api: Scripting API for Aseprite
Examples Aseprite Script Examples
Scripts collection Aseprite Scripts Collection
I hope this helps.
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