[dan-robertson]

Emacs has this plugin (well not a plugin because it’s built in). One can select an input mode. Some translate directly (e.g. a Greek input mode where typing a gives an alpha). There is also a TeX mode like what you describe and another mode which is more concise (e.g. &a for alpha, where the means Greek). The difference from what you described is that instead of some escape sequence for these special prefixes, they are just never converted if they don’t match and you move the point away.