Victory!
I succeeded installing fmcb using Medal of Honor Rising Sun.
I replaced all elf files in the MOHRS directory in the root of the dvd. This includes EAGLRM.O which is also an elf file.
The swap requires a bit of timing and needs to be done just as the playstation 2 logo disappears. I succeeded on the second try. After the swap uLaunchElf should start automatically.
Game: Medal of Honor: Rising Sun (Platinum Edition) - PAL - SLES_518.73
Console: PS2 slim - SCPH-75004
Memcard: Original Sony 8MB
uLaunchElf: Version 4.12 - Compressed
Disc: Philips DVD+R 1-16x speed
For the file replacements i overwrote the start of each original elf with the uLaunchElf elf so my new file had the same size as the original, so i didn't need to change the size in the TOC. Probably it also works using the regular method.
Note this game also contains some elf files in the NETGUI directory, however replacing these didn't work for me as an additional disc check is done before these are launched (you get the red screen).