A player option to refuse could be added, with a Free Allied Cores casus belli awarded to UK, although it is very unlikely to be successful if the UK acts on this casus belli.

On that note, it may worth trying if the attacker_goal subcommand can support both a target and a state_province_id. This is what ensures that a war targets a specific state, Northern Malaya in this case:

This is what ensures that a war acts on behalf of a specific country:

That is, in the above the European is declaring on Egypt to liberate Ottoman cores. Same as if they had selected the Ottoman Empire in the list of valid liberation targets.

So for a CB like Free Allied Cores (or Free Peoples) that targets a state on behalf of a country, why not try the following:

war = {
    target = EGY
    attacker_goal = {
        casus_belli = free_peoples
        state_province_id = 1391
        country = TUR
    }
}

Which, of course, is attempting to liberate Northern Malaya from Egypt on behalf of the Ottomans. I’ll give it a whirl and report back (though perhaps with a more reasonable target).

It totally works. Bonus: unusual war name.

Propping up Brittany! That is nice. I think this is a great option, although I think it probably is best performed as part of a flavor upgrade for Hudson's Bay and Canada. The reason that I suggest this is that Hudson's Bay never gets an event for the Rupert's Land Act. Therefore, the event would need to be modded to send an event to RPL to give it the option of refusal. This sounds like something that should be done in a separate file, as we are doing for PlatineWar.txt. Since it sounds strange to have a file with only one event in it, I suggest making the refusal be part of a flavor upgrade project.