You can use the following rule:
ignore sub e' slash' o' @AllLetters;
sub e' slash' o' by e_slash_o;
Here, no space glyph is needed (which – as you said – can be problematic). Instead, the substitution rule is preceded by an ignore rule. The ignore rule says that the pattern e' slash' o' should not be matched if followed by a letter (@AllLetters).
Glyphs can generate the @AllLetters class automatically for you. Click the plus + button next to the Classes heading in the Features tab and select AllLetters from the menu:
