But there is no signs of CHDK on display.
You need to port everything that is related to the bitmap and stencil buffers (lib.c and stubs_min.S). The bitmap buffer stuff is probably easier. IIRC only the first stencil buffer's address can be found in the sx280 firmware, the second starts right after the first one (the buffer size should be equal to the maximum supported resolution). I actually used RAM dumps to visually identify the buffers.
See also update_screen_dimensions() in lib.c and the firmware routine I mention there.
If you run spytask without adjusting the display related fw variable addresses, you're effectively writing memory you're not supposed to.
edit:
"stencil buffer" = stores opacity for all pixels of the bitmap overlay (8 bits per pixel)
"bitmap buffer" = yuv buffer for bitmap overlay, two adjacent pixels share the same chroma
edit2:
below post was answered in PM