Can non-Barbarians take the Whirling Frenzy alternate class feature? For example, if I were to take two levels in the Half-Orc Paragon class, can I replace rage gained this way with Whirling Frenzy?
What if the ability to fly into a rage is gained from the Druidic Avenger class, wich is a variant class itself?
Yes.
Rather than being a variant to the Barbarian class itself, like Totem Barbarian for example, Whirling Frenzy is explicitly listed as a variant to the Rage class feature.
Consequently, any character with the Rage class feature can substitute it for Whirling Frenzy.
Counterarguments:
That said, the text only mentions barbarians. Furthermore, it also substitutes indomitable will by evasion at 14th. Both of these points can be used to rule against the above.
For the former it can be argued to just be the default class associated with rage, similar to the expression rogue's evasion, when other classes can have evasion as well. The latter can be trivially adjusted for other classes that do or do not get indomitable will at some point in their progression.
Whirling Frenzy is a Class Feature Variant, which are “variant versions of several of the iconic class features common to the character classes,” and replace features rather than being a specific variant on an entire class.
So a half-orc paragon could select Whirling Frenzy, yes. The druidic avenger’s slightly more dubious, but most DMs allow that kind of “chaining.”
Ultimately, as variants, these things are entirely up to the DM, especially implementation details like who can or cannot have them. Most DMs I’ve played with have been very lax about it, and I personally think that’s a very good thing for the game (more options and variety is good), but you’ll have to talk to your DM.
For what it’s worth, I am aware of no combination or series of “chained” variants that will cause a problem, balance-wise.
