HDMI output only supports a subset of all modes documented in README:

• TEXTMODE_80x25_COLOR (rendered as 320x150 with chunky 4x6 characters)
• CGA 320x200x4 including a "BW" variant
• Composite 160x200x16 including a "force" variant
• TGA 160x200x16
• TGA 320x200x16
• VGA 320x200x256 (including a "x256x4" variant, probably the famous "X mode")
• EGA 320x200x16x4

Booting a floppy: 

Grab a compatible floppy, I used the file from MS-DOS 3.3 - Working Copy With Utilities direct link is DOS 3.3 Working.img

Create a directory "XT" on the SD card

Put this in the "XT" directory and rename to "fdd0.img"

It should also be possible to create the following image names: fdd1.img hdd0.img hdd1.img

I made a bootable floppy based on

https://archive.org/download/msdos_VGAFUN_shareware

It's temporarily at 

https://media.unpythonic.net/emergent-files/sandbox/mario-joust.img

put this on the device as fdd0 and you can run a bad port of joust or mario (the arcade ones with the pipes)

my fave, commander keen, doesn't work. It probably uses an unsupported EGA mode such as 640x350.

Some images from the makers of pico-286 at https://murmulator.ru/pico-xt: https://file.murmulator.ru/data/flash-cross/pico-xt/hdd.img.zip  https://file.murmulator.ru/data/flash-cross/pico-xt/Images.zip

unzip hdd.img to (SD CARD)\XT

take dos500a.IMA from Images.zip and rename it "fdd0.img" (must remove IMA extension and change it to img)

now you can go "c:" "cd games" "dir" and see if any games work (most don't)

keen4c: works! CGA graphics only

"qbasic" - "gorilla.bas" works!

"cd sbplay" "test" works (plays a wave file)

"cd adlib" "jukebox" works, plays polyphonic audio

"cd games\kq4" - kinda runs, audio sounds bad (clipping?), video is CGA(?) or palette is wrong

"cd games\sq3" - runs better than kq4, still CGA, audio still not great

"cd games" "digger" - runs OK, but constant background hum

test drive - select EGA mode. seems to work? but with vertical artifacts everywhere. didn't figure out controls. a/z shift & repeatedly tapping left/right/up/down to accelerate/turn/etc?? (may be that kbd repeat function is not implemented)

nu - norton utilities - kinda works but all text flashes!

dosmid - test.bat - sounds baaaaaaad

lemmings - graphics scrambled 

wolf3d - graphics scrambled

hero quest - promising intro screen with cool audio but I couldn't successfully enter the game

midiplay - doesn't work (plays brief static)

minix 286 2.2: doesn't start booting (black screen w/cursor)

minix 286 2.0: glitches after pressing "=" to boot http://download.minix3.org/previous-versions/Intel-2.0.0/i86/ doesn't start

https://archive.org/details/xenix_2_2 reaches boot prompt, but can't continue (tried instructions at http://bitsavers.informatik.uni-stuttgart.de/pdf/sco/system_V_2.x/2.3.2/XG-5-16-88-5.0_XENIX_2.3_Installation_May88.pdf involving fd(4) and so on)

mapdrive works sorta!

This is the way to put files without placing them inside a floppy or hard drive .img. You still need a bootable "A" (or maybe C?) drive though! and you have to put settings in config.sys

• put some files and folders in /XT/MAPDRIVE on the SD card
• use DOS 5.0 (possibly any DOS after 3.3; but probably not 3.0??)
• set LASTDRIVE=H in config.sys (must reboot after changing config.sys)
• run mapdrive.com (including from autoexec.bat)
• cd h:
• dir

Note: "dir h:" doesn't work and I couldn't determine why (that's what this screenshot is showing)

most games are glitched

Used resources

• uart debug on pin a5
•  HDMI:
  • PIO0 SM??,?? (PIO via macros PIO_VIDEO, PIO_VIDEO_ADDR; SM via claim_unused_sm)
  • DMA: 4 channels (via claim_unused_channel)
  • DMA_IRQ: 0 (via macro VIDEO_DMA_IRQ)
• Audio:
  • PIO0 SM?? (PIO via i2s_get_default_config, SM via claim_unused_sm)
    • can probably go on PIO0
  • DMA: 1 channel (via claim_unused_channel)
  • DMA_IRQ: none(?) completion via dma_channel_wait_for_finish_blocking
• pico-pio-usb:
  • PIO 1 SM 0/1/2
  • DMA: ??
  • DMA_IRQ: ??