I got one of these pie screens 2.8 inch Touch Screen for Pi zero - raspberry pi wiki.
It runs from the GPIO with just some files added into the boot patition and a few lines in the config.txt
dtparam=spi=on
dtparam=i2c_arm=off
#dtoverlay=mzts-28
dtoverlay=ads7846,penirq=27,swapxy=1,xmin=200,xmax=3850,ymin=200,ymax=3850
display_rotate=3
dtoverlay=mzdpi
overscan_left=0
overscan_right=0
overscan_top=0
overscan_bottom=0
framebuffer_width=640
framebuffer_height=480
enable_dpi_lcd=1
display_default_lcd=1
dpi_group=2
dpi_mode=87
dpi_output_format=0x07f003
hdmi_timings=488 0 228 8 1 640 0 4 2 1 0 0 0 60 0 32000000 3
I did not manage to be able to make the pie switch the output properly.
When i connect HDMI while the pie screen is connected, i get some kind of signal but the TV stays black.
If i set display_default_lcd=0 the pie screen stops working allready.
I also tried the HDMI force commands and other 480p timings all without luck.
I also do not understand how i could fix the overscan on both displays with just one config.