[luatex] Inserting a "dummy" node

[Javier Bezos]

Is there a "canonical" way to insert a "dummy" node passing some
data to the list, so that it can be used when processing it?

I'm writing a new feature for babel, and while I've discarded
this approach (an attribute is clearly better), I'm still
wondering if there is a preferred method. I thought a user
whatsit was the solution, but if I've understood correctly,
it cannot be added by TeX (only by lua).

Javier