That will work. If you want to stay true to the OpenType specification, use uppercase letters for the tag, e.g. CUST for custom. Lowercase letters are reserved for registered axes (currently wght, wdth, slnt, opsz, ital).
It is highly unlikely, but at one point in the future, (lowercase) cust may be registered for something, and applications may treat it differently than other axes, and that could potentially break your font.