Since one of the last few DocBook versions, using e.g. <keycap function="control"/> is enough to generate a keycap with the text "Ctrl" in it.
While this function in principle does work with <keycap function="option"/> too, in this case it only gives a keycap with "??" as its label. My expectation would be that it either output the sign "⌥", the word "Option" or a combination of both. More information about this Apple-specific key.
All that is necessary to make this work, is an adaptation of the appropriate l10n files.