Cast a spell, limited to a cantrip, would probably be fine - the cantrip baseline is lower than the attack baseline and it scales less dramatically than extra attack. EB is a cantrip outlier, of course, since it acts more like Extra Attack, but, it is still something the warlock can do every round, their at-will baseline.
That is, if we're talking giving up an action to grant attacks.
Giving up an attack to grant an attack, when you, yourself, get Extra Attack is different, because now /you/ are providing scaling. That can work logically enough with other classes that also scale via Extra Attack, at a similar rate - only the Fighter would start to look at you askance at high level, that way. By the same token, it could work with EB, and the right wording - maybe something about a single attack roll? But, as soon as you start using that scaled attack-granting on someone who scales their at-will baseline in some other way, it becomes more questionable.