Lomaster san,
A bit late to the party, but I think in this case it's "de" but more because of what you're saying in the sentence, rather than whether hanei sareru takes a "de" or a "ni."
by means of/conveyance = de hyougen sareru
in (almost like a direct object) = ni hyougen sareru
For example (artistic paintings and expressions)
この世の無常がこの表現で反映されています。
kono yo no mujou ga kono hyougen de hanei sareru = the transient nature of the world [ga] is reflected *by* [de] this expression
hyougen *de* = by means of/due to/through (this) expression
taking the "hyougen" completely out of the sentence...
kono yo no mujou *ga* hanei sareru = the transient nature of this world is reflected
(by what? add "hyougen de" back in and you see it's reflected by "this expression")
mujou is the subject (marked by particle "ga") and hyougen is the conveyance (how, marked by the particle "de") something is reflected.
But...to add a wrinkle, there's no "where/in" (e.g. a specific place where something is reflected) in the sentence. But, if the sentence were:
kono yo no mujou ga (kono) hyougen ni hanei sareru = transience is reflected *in* [ni] this expression (not "by" this expression, but "in" this expression)
So, then there would be a ni between hyougen and hanei sareru.
Or, to extend the sentence (similar to yours) we can use de and ni. "De" after hyougen to state "by means of" and "ni" after kono kaiga (in this painting) to state exactly where...
kono hyougen de yo no mujou ga kono kaiga ni hanei sareru = _by means of this expression_ [hyougen de], the transient nature of the world [mujou ga] is _reflected in_ this painting [kaiga ni hyougen sareru].