As z7sg Ѫ (what a tremendously untypable name!) says, ‘smeller’ is the most straightforward answer.
However, ‘smeller’ fails on some levels, since it is in style and register comparable to ‘seer’ or ‘viewer’ (disregarding the air of clairvoyance that ‘seer’ has gained in English). ‘See’ and ‘smell’ are on par, but ‘see’ and ‘behold’ are not: one is colloquial, neutral, and default; the other is formal, archaic, stylised, and quite uncommon in comparison.
Alas, there does not seem to be a similarly stylised and formal synonym for ‘to smell’, so a direct parallel to ‘behold’ does not quite work. But since the sense of smell is quite commonly known to be more formally termed the olfactory sense, and the act of smelling thus also termed olfaction, I would suggest that a better formal and stylised term for someone who smells would be an olfactor.
This word does indeed exist; Wiktionary defines it as:
A smelling organ; a nose.
The OED, however, adds a secondary, rarer, meaning that squares with this (italics mine):
A nose; an organ of smell; (also) a person who perceives smells.
Indeed, there is even a quote from 2002 that shows that you are not the first to come up with this punny parallel to the idiom (nor I with the word):
City Jrnl. (N.Y.) (Nexis) Summer, The smell of urine is in the nose of the olfactor.
So I feel quite safe in suggesting:
The fragrance is in the nose/nostril of the olfactor.