Have a look at the readme (boot/imgmount commands).

They don't seem to be mentioned in any of the README files provided.

Not even on lines 567-643?

Maybe I am a really lucky guy, with a special readme that mentions those commands? Or maybe I just know how to read?

567 IMGMOUNT 568 A utility to mount disk images and CD-ROM images in DOSBox. 569 570 IMGMOUNT DRIVE [imagefile] -t […]
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567 IMGMOUNT
568 A utility to mount disk images and CD-ROM images in DOSBox.
569
570 IMGMOUNT DRIVE [imagefile] -t [image_type] -fs [image_format]
571 -size [sectorsbytesize, sectorsperhead, heads, cylinders]
572
573 imagefile
574 Location of the image files to mount in DOSBox. The location can
575 be on a mounted drive inside DOSBox, or on your real disk. It is
576 possible to mount CD-ROM images (ISOs or CUE/BIN) as well, if you
577 need CD swapping capabilities specify all images in succession.
578 The CDs can be swapped with CTRL-F4 at any time.
579
580 -t
581 The following are valid image types:
582 floppy: Specifies a floppy image or images. DOSBox will automatically
583 identify the disk geometry ( 360K, 1.2MB, 720K, 1.44MB, etc).
584 iso: Specifies a CD-ROM iso image. The geometry is automatic and
585 set for this size. This can be an iso or a cue/bin.
586 hdd: Specifies a harddrive image. The proper CHS geometry
587 must be set for this to work.
588
589 -fs
590 The following are valid file system formats:
591 iso: Specifies the ISO 9660 CD-ROM format.
592 fat: Specifies that the image uses the FAT file system. DOSBox will attempt
593 to mount this image as a drive in DOSBox and make the files
594 available from inside DOSBox.
595 none: DOSBox will make no attempt to read the file system on the disk.
596 This is useful if you need to format it or if you want to boot
597 the disk using the BOOT command. When using the "none"
598 filesystem, you must specify the drive number (2 or 3,
599 where 2 = master, 3 = slave) rather than a drive letter.
600 For example, to mount a 70MB image as the slave drive device,
601 you would type:
602 "imgmount 3 d:\test.img -size 512,63,16,142 -fs none"
603 (without the quotes) Compare this with a mount to read the
604 drive in DOSBox, which would read as:
605 "imgmount e: d:\test.img -size 512,63,16,142"
606
607 -size
608 The Cylinders, Heads and Sectors specification of the drive.
609 Required to mount hard drive images.
610
611 An example how to mount CD-ROM images:
612 1a. mount c /tmp
613 1b. imgmount d c:\myiso.iso -t iso
614 or (which also works):
615 2. imgmount d /tmp/myiso.iso -t iso
616
617
618 BOOT
619 Boot will start floppy images or hard disk images independent of the
620 operating system emulation offered by DOSBox. This will allow you to
621 play booter floppies or boot other operating systems inside DOSBox.
622 If the target emulated system is PCjr (machine=pcjr) the boot command
623 can be used to load PCjr cartridges (.jrc).
624
625 BOOT [diskimg1.img diskimg2.img .. diskimgN.img] [-l driveletter]
626 BOOT [cart.jrc] (PCjr only)
627
628 diskimgN.img
629 This can be any number of floppy disk images one wants mounted after
630 DOSBox boots the specified drive letter.
631 To swap between images, hit CTRL-F4 to change from the current disk
632 to the next disk in the list. The list will loop back from the last
633 disk image to the beginning.
634
635 [-l driveletter]
636 This parameter allows you to specify the drive to boot from.
637 The default is the A drive, the floppy drive. You can also boot
638 a hard drive image mounted as master by specifying "-l C"
639 without the quotes, or the drive as slave by specifying "-l D"
640
641 cart.jrc (PCjr only)
642 When emulation of a PCjr is enabled, cartridges can be loaded with
643 the BOOT command. Support is still limited.

Right. Sorry, the text search must have been case-sensitive (even though it said it wasn't!).