Can you maintain concentration on non-spell abilities (that require it) while raging?

Question

There are a few class features in the game that require concentration, but are not spells: A Trickery Cleric's Invoke Duplicity, a Glamour Bard's Mantle of Command, a Graviturgist Wizard's Adjust Density, etc. They are all phrased in a similar way concerning concentration:

for 1 minute, or until you lose your concentration (as if you were concentrating on a spell).

Going into a rage stops you from keeping concentration on a spell:

If you are able to cast spells, you can't cast them or concentrate on them while raging.

But, if a multiclassed character (with one of the above subclasses, plus barbarian) uses such a concentration-based feature then goes into a rage, are they able to maintain concentration on that feature?

For example: Timmy the Trickery Cleric/Barbarian uses Invoke Duplicity, then enters a rage. Is Timmy able to continue concentrating on Invoke Duplicity?

Answer 1

No, you can't, because they require concentrating "as if" on a spell

As you note, the barbarian's Rage feature says:

If you are able to cast spells, you can’t cast them or concentrate on them while raging.

The Trickery Domain cleric's Invoke Duplicity Channel Divinity option says (PHB, p. 63; emphasis mine):

As an action, you create a perfect illusion of yourself that lasts for 1 minute, or until you lose your concentration (as if you were concentrating on a spell).

Because you need to maintain concentration on the illusion "as if you were concentrating on a spell", anything that prevents concentrating on a spell also prevents you from concentrating on something "as if" it were a spell.

Rules designer Jeremy Crawford unofficially confirms this interpretation in this January 2018 tweet, in response to a now-deleted question that specifically asks about the Invoke Duplicity feature:

Can you concentrate on Invoke Duplicity while Raging?

You can't concentrate on a spell while you rage. This prohibition applies to abilities like Invoke Duplicity that require you to concentrate on them as if they were spells.

Rage specifies that you can't concentrate on spells while raging; the same restrictions apply to things that require you to maintain concentration "as if you were concentrating on a spell". As far as I know, all official non-spell features that require concentration in this way specify that you do so "as if you were concentrating on a spell", and so Rage prevents the character from concentrating on any of these features.