[Disclaimer: I do not know enough (by far) about the inner workings
and secrets of these printer-consoles, so please correct me/send advice,
if there are better solutions!]

On 3215/327x things are dramatically different from everywhere else.
You'll have to live with some severe limitations:

0. Interactivity is quite limited.  You'll need to "blindly" type,
   most of the time, to see the effect only on "submission" ([Enter]).
   In edit and shell mode it's sometimes useful to insert underlines
   just to see, where the curser (AKA "point") is.  (BTW, 3270 is _much_
   better at displaying/refreshing grub2 screens than 3215.)
1. No cursor-movement-, alt-, meta-, and control-keys (like [ESC]).
2. To work around the lack of control-keys, the "[^][C]-sends-interrupt"-
   trick is extended to translate sequences of caret followed by character
   to the respective control-character.  In the following this sequence
   of two keystrokes is referred to as '^c' instead of that somewhat balky
   [^][C].  Thus an [ESC] keypress can be generated with '^[' ("caret"
   followed by "opening square bracket").
3. If a caret itself is needed, send one on it's own (i.e. a solitary [^]
   followed by [Enter] -- or use '^^'.
4. No '[Enter]', because it can't be avoided on *any* input.
5. If you still need one to arrive at the application, you may either
   press '[Enter]' *twice* (one empty line, sort of) or add '^j' to your
   input.  In menu mode '^f' works as well (see below).  But using "empty
   lines" does now work very reliably, so explicit control sequences
   are to be preferred.  This has the additional advantage, that combined
   sequences can be sent, e.g. to exit from 'grub2-emu' without doing
   anything, you can simply type 'cexit^j' and submit that with [Enter].

Common Substitutes:
 '^j'`	=> [Enter]	"engage"
 '^['	=> [ESC]	"abort" / return to previous "state"
 '^i'	=> [TAB]	try completion (in edit & shell mode)

Available Keys in Menu Mode:
 '^a'	first entry		'^e'	last entry
 '^p'	previous entry		'^n'	next entry
 '^g'	previous page		'^c'	next page
 '^f'	boot selected entry/enter sub-menu (same as '^j')
 'e'	edit selected entry	'c'	enter grub-shell

Available Keys in Edit Mode:
 '^p'	previous line		'^n'	next line
 '^b'	backward char		'^f'	forward char
 '^a'	beginning of line	'^e'	end of line
 '^h'	backspace		'^d'	delete
 '^k'	kill (to end of) line	'^y'	yank
 '^o'	open line		'^l'	refresh screen
 '^x'	boot entry		'^c'	enter grub-shell

Availble Keys on Command Line Mode:
 '^p'	previous command	'^n'	next command (from history)
 '^a'	beginning of line	'^e'	end of line
 '^b'	backward char		'^f'	forward char
 '^h'	backspace		'^d'	delete
 '^k'	kill (to end of) line	'^u'	discard line
 '^y'	yank