Most people don’t seem to particularly care, where exactly on their hard drive a software stores its data, but yes, I get the point. I’m also kind of curious when such questions pop up, just not quite as judging.
Maybe ccvault.io would suit you? They have a web-hosted and a chrome-like browser plugin version and offer to export the wallet in a JSON file that can be imported again to restore.
(Sounds like day-to-day the secrets are additionally stored somewhere inside the browser storage just like for Yoroi, but it goes a long way to your demands, I think.)
If your system is compromised enough to have a key logger or screen grabber, the secrets are not much safer in a file, I think, even if it’s on a USB drive.
I’d also very much like to have a software implementation of it. Should be possible. I think, everything they do is public. A seed put into such a software would, of course, instantly lose its “hardware wallet security”, but you would keep access and could decide to buy a hardware wallet from another vendor or switch to software wallet or whatever.