Yes it is still possible, but note that both HttpPostRequestDecoder and HttpPostRequestEncoder have the HattpDataFactory as argument, which should be setup to use (or not use) this RFC conforming rule, since this is where (HttpDataFactory) this rule has to be setup.

When I wrote this helper method, I feel like a specific class should be setup (a new class helper) such that it can easily be reused, maybe in the DefaultHttpDataFactory class, or a new one (which name ?).
Note that I'm not so happy that someone could implement a "bad" conforming service (contrary to any good practices in IT), so the reason to not make it "so" easy, and in particular not the default one. So putting this as an argument to the Encoder/Decoder as explicit argument is somehow not particularly intended.

Moreovern put it in Encoder/Decoder will break the underlying implementation where the factory is responsible of the constructor (factory), while the encoder/decoder is not. It could lead to an heavy change (I did not take time however to see if it is particularly difficult or not).

So let me know what is your preference...
Cheers