The letters ä, ö, ü in German were written (e.g., in Gutenberg's Bible) with the respective vowels that had a tiny e printed right above them. Are these characters available in Unicode? They looked something like:
e e e e e e
A O U a o u
If they are not available in Unicode as single glyphs, perhaps they can be "produced" using Unicode control characters? For example, I thought of using Unicode character 1d49 ("MODIFIER LETTER SMALL E"), but the glyph does not appear above the previous vowel, but on its upper-right.
It's not a "control code" but a Unicode Combining Diacritical Mark, U+0364.