Can't be any other way than the UEFI legacy mode / Compatibility Support Module (CSM) / BIOS emulation? Let's call it the CSM, since they made up that acronym just so we have a name for it.
As for full USB read/write under DOS, this is courtesy of needing of BIOS (emulated or not) needing to read USB drives in the first place ; otherwise you wouldn't be able to boot from USB, or other features. DOS will read/write the USB drive you booted from.
As for CHS, this would give you the old drive limit of slightly less than 8 GiB. LBA took care of this and it might be the BIOS's job, not DOS (I don't know). A version of DOS that supports fat32 will help of course (prior to 98SE, a buggy fdisk was bundled, partition size above 64GB rolls over). You will certainly be able to use any drive up to 128 GiB / 137 GB, quite possibly up to 2 TiB. (don't make a fat32 partition that big if you worry about cluster allocation size)
Sound card support is the only thing really missing to have some fun IMO. I did see that mpxplay (a music player) includes drivers for certain cards and sound chipsets (some Intel, VIA, other in there but no Realtek) ; I never tried it but with supported hardware it might actually be useful.