Unfortunately the antipattern of Actionable returning a mutable collection from getActions, coupled with questionable and inconsistent behaviors of its subtypes, has made it seemingly impossible to compatibly change how Actionable.getActions works. The same is true of nonfinal Actionable subtypes like AbstractItem which do not themselves currently override Actionable methods.
The only compatible course is to introduce a new method which would include entries from a TransientActionFactory, though then lots of code would need to be updated to call it: basically anything which calls getActions and treats the result as read-only.
(And User and View are not Actionable despite having comparable methods and transient factories! So they cannot use any new facility.)
Making Actionable understand transient actions is actually a good idea, because transient actions in general are typically done as a hack.
Although it probably implies calling transient action factory on every page rendering.