Is it possible to inflict banes on allies or invoke boons on enemies? I can think of several situations, where this would be very flavorful, but it has its problems. Let me begin with some examples.
Your perception is unprecedented, which is why you manage to see the blade trap long before your party member. However, there is no time to shout a warning, so you let yourself fall down and take her with you. Within the mechanics, you could inflict the knockdown bane on your ally to save her from greater harm.
You are in a melee fight with a lot of enemies. Their leader is well protected by his many bodyguards. In order to disperse the crowd, you decide to place an aura of persistent damage on the leader, which forces his bodyguards to decide between letting you through to him or taking a lot of damage.
You could make the strongest opponent insubstantial to remove him from the equation while dispatching the less dangerous foes.
You could force move an ally out of a dangerous situation, without provoking any attacks of opportunity (this is possible with the teleport boon, so why should it not be possible via forced move bane?)
In addition to bestow invisibility on your party, you might want to prevent them from emitting noise. This might be possible via the silenced bane.
As you see, there are some applications for creatively invoking boons on enemies or inflicting banes on allies. I didn’t find anything in the core rules, which would prohibit such a use. Only some banes or boons read in their description that “you or an ally” can do something. However, this is in the description and not in the effect text, which leads me to conclude that it is not a rule.
However, there are problems with this. A druid might decide to shapeshift enemies and polymorph allies instead of the other way around, because of how the limitations work. As far as I can tell, there are no limits to how strong a polymorphed creature can be, there is only a limit to how weak it can make the target. And shapeshifted characters lose all their extraordinary attributes, which would make it an ideal application for an enemy mage. Also, as a bane the target could choose not to resist and keep the form indefinitely without the need to sustain, while the decision about whether or not to keep the shapeshift on lies with the caster, instead of the target. Here, the designers obviously meant polymorph for enemies and shapeshift for allies, and balanced them as such.
So… What do you think about it? Do you forbid this kind of thing from the beginning or do you decide on a case by case basis?