It's been some way to go, but now I'm in control of my #Librem5: I can access the boot prompt.
I'm used to be able to do so to debug boot problems (in those rare cases) using grub.
Nice side effect: this version of u-boot (build to support uart2 instead of uart1) does support booting from sdcard.
Best part of it: No need to open the L5 at all and thereby low risk of breaking anything.
Would be nice to find a cheaper device than DELL TB16 to trigger thunderbolt alt mode...