No problems with putting the badge on the HR, especially if your Grandad had the kookaburra installed on his HR. Your brother would probably think it is a great present.
I would suggest that the Kookaburra predates the HR by about 35 years. Can't give you a date for this mascot exactly, but I've seen them on pre-World War 2 cars- early to mid thirties.
Would think it would sell for $150-$250 if you are that way inclined.
As for rarity value, they are probably rare in South Australia or Victoria, but there are a few about in Queensland of course. :PUnfortunately wearing the badge on the bumper bar doesn't provide for cheaper RACQ insurance or membership.
Your brother should send them a bill for advertising when he installs it on the HR.
Happy motoring,