Please don't take this the wrong way, but isn't 24P your backup in that situation? For sure, respond Rescue where a pump is required, but if Rescue is already tied up and you have crews available for a second appliance/job then send 24P. Heck, 24P is good enough for a lot of brigades as their primary urban pump, it should be good enough for Mt Barker as a backup pump??
I also do realise that MtB have been accustomed to a dedicated pump as well as a seperate Rescue, and have had to back pedal to their original setup of 24P/24/Rescue, and no one likes going backwards - but at least their current rescue is MORE than capable as a Pumper.
I just think that a lot of noses will be put out of joint if MtB get a second type 2 before some other brigades can even get their first one. My two cents.