Good day, Elmar! I've just tested your Plop Boot Manager 5.0.15 as a part of coreboot+SeaBIOS opensource BIOS
by adding your plpbt.img floppy to my fresh coreboot.rom build with the following command:
./coreboot/build/cbfstool ./coreboot/build/coreboot.rom add -f ~/Downloads/plpbt.img -n floppyimg/plpbt.lzma -t raw -c lzma
I am happy to tell you that Plop Boot Manager seems to be working great - at least successfully boots Linux from USB flash drive!

However, I am really concerned by that Plop Boot Manager source code is not open:
it could be hard to justify keeping a closed source proprietary software as a part of opensource BIOS
for security considerations: theoretically any closed source software could contain the backdoors,
and having it as a part of BIOS is a permanent security risk that I would like to avoid if possible.
Elmar, please could you release the source code of Plop Boot Manager?
Also I've just learned that you are going to release a new Boot Manager soon.
Don't know if it would be open source, but it would be really nice if you could
at least liberate the source code of plpbt-5.0.15 which came out almost 6 years ago
P.S. if you became curious about this opensource bios, please visit the coreboot board status page, to check
if your motherboard is supported , or maybe you could get one? personally I am using AMD Lenovo G505S laptop :
it has a powerful quadcore A10-5750M CPU
without AMD PSP hardware backdoor
(an equivalent of Intel ME backdoor),
supports 16GB of RAM and could be found in good condition for just $100-$150. Great choice for security conscious people
Best regards,
Mike Banon,
opensource firmware enthusiast