One of the things i did note with the casting resin (and to be fair the destructions do warn of this) is that it has a tendency to pull back from the edges as it cures. Where there is a a roughened edge for the acrylic to bind to this can lead to cracking.
Due to the smooth finish on some parts of the void i was filling (a product of the NZ Matai) it actually pulled away a little as it would do with a plastic mould. It isn't really a game breaker, I've since wicked some CA into those wee gaps and then routed the acrylic block down to the depth I needed and there seems to have been no movement at all. it machines well, if somewhat messy and stinky and the finish it coming up well under sanding. I shall have to look for a project to use your method on now!