I’m not sure what you’re trying to do, but if your intent is simply to allow the player to continue with input that was interrupted by a real-time event, this capability is built into glk. This thread from a few weeks ago should be of some help:
https://intfiction.org/t/inform-the-reliques-of-tolti-aph/67/1
On the other hand, if what you really want is character-by-character control over line input, you can in fact get that by recreating the line interface using character input, and it wouldn’t be all that hard to do–but it’s not to be recommended either. You won’t have a cursor, for one thing (though you can imitate one, I suppose), and your players will lose the ability to paste input to the command line.
–Erik