I recently had to modify the kernel command line of a machine that I accessed with a serial console. GRUB displayed the following message explaining that I had to "press Ctrl-x or F10 to boot".
Minimum Emacs-like screen editing is supported. TAB lists completions. Press Ctrl-x or F10 to boot, Ctrl-c or F2 for a command-line or ESC to discard edits and return to the GRUB menu.
Unfortunately, pressing F10 in minicom was equivalent to hitting ESC: my changes were discarded and I was sent back to the GRUB menu.
It turns out that the following keystroke sequences allow to send F10 with minicom: ESC+O+Y or ESC+0.