@greed
you can try with hdmi cvt in the config.txt :
https://www.raspberrypi.org/forums/viewtopic.php?t=24679
something like this, with your own resolution :
hdmi_cvt=1920 240 60 1 1 0 0
info on numbers :
hdmi_cvt=<width> <height> <framerate> <aspect> <margins> <interlace> <rb>
width width in pixels
height height in pixels
framerate framerate in Hz
aspect aspect ratio 1=4:3, 2=14:9, 3=16:9, 4=5:4, 5=16:10, 6=15:9
margins 0=margins disabled, 1=margins enabled
interlace 0=progressive, 1=interlaced
rb 0=normal, 1=reduced blanking
and you need to setup the boot in this custom mode, in the config.txt :
hdmi_group=2
hdmi_mode=87
(custom mode is on mode 87)
in recalbox.conf, you should put
global.videomode=default
or
global.videomode=DMT 87 HDMI
I think the default should work.