Just a small suggestion:
If entering =| (i.e. without specifying a glyph name) for the sidebearing of a glyph that would be mean “use the opposite of this glyph”.
So, instead writing =|backslash, I could simply write =| when setting the SB of backslash. This case happens to me a lot and the simplified syntax would be quite convenient. When referencing the opposite SB of another glyph one would stil have to use the explicit syntax.