Unfortuanently it is something we can't offer at the moment. But due to the strong interest, we've been considering creating a redistributable, "locked-down" versions of Odin that people can use to create inspectors in products they distribute, but that doesn't let people who have the distributed product use Odin in their own non-product-related custom inspectors.
It would likely take the form of a special download from our website and would bypass the Asset Store completely. We still haven't settled on options, though, as it's early days yet.
Apart from the financial/legal model, there are also a bunch of tech issues to work out, in terms of how to make it all easy on everyone, and still allow people who have your products to install "actual Odin" in their own projects, without that conflicting with your distributed "locked-down" version. Different "locked-down" versions also have to be able to work in the same project, if several different assets are using Odin to draw their inspectors.
It'll probably be a few months before anything like this happens though.
This, of course, is only relevant for the inspector part of Odin Inspector. You can, and we highly encourage you to, use the open source serializer in any of your own asset store plugins. You can find the repository on github here: https://github.com/TeamSirenix/odin-serializer