Honestly I have no idea how serde works and if what you want to do is even possible to do efficiently (without writing some custom regex that matches all valid sections), but here is what I would do, and maybe improve on it later on if benchmarks indicate this is a good place to optimize.
An easy solution would be testing every offset until serde tells you there is something valid at that offset. Certainly not a very fast solution, even quastionable if it won't find something inside your garbage, but easy to implement for sure. If performance is of no (major) concern I would at least try it..