An IMG file typically is just a raw byte-for-byte copy of every sector on the drive, starting with the MBR+partition table.

Windows NT actually has device nodes, (like Device\Harddisk0\Parition0\etc.), they just aren't conveniently put on the file system for use with tools like dd. It doesn't even have to be dd; even cat /dev/deviceXYZ > file.img works for this purpose too. You can see such device nodes in Sysinternals WinObj. Looks like there is a Sysinternals disk2vhd also, but looking at the screenshots, it isn't clear whether it images an entire drive or just one partition at a time.

I haven't tried WSL but good old fashioned Cygwin had /dev emulation at one point.

One thing to keep in mind is if Windows will pounce on the drive when it sees it, scribbling Thumbs.db files in each directory and such.

If WSL doesn't map Windows drive letters to /dev drives, what good is it?

You can use this:
https://hddguru.com/software/HDD-Raw-Copy-Tool/
Or TestDisk:
https://www.cgsecurity.org/wiki/TestDisk_Download

Google "dd for windows". I've used it in the past to dump the contents of a compact flash in a PCMCIA adaptor to hard drive on my employer's laptop. (My normal imaging method for an IDE drive is to plug the drive into a Linux box using a USB/IDE adaptor then use dd to do the dirty work, ended up using dd for windows because the only PC I had with a PCMCIA slot was the work laptop).

Well, hdd raw copy did dump it, but now I can't really mount and use the drive under DosBox-X. The boot drive has an old version of Partition Magic installed for multi-boot, compounded by it is a drive that uses a CHS of 3158,16,63 so BOOT won't work (IMGMOUNT as a drive number will work, but still can't boot from it).

Am I missing something?

OK, so the BIOS supports LBA (probably the 512MB version). But that boot drive has EZ-Bios (Maxtor I think) installed on it, so it shows as 789, 64, 63 CHS. The bios is copyright from 1992 - 2001 Storage Soft.