Can't you just lift the motor off and take it home to do .

no to big job to take of and take home but i was thinking it would work with container in dinghy to catch oil but dealer is doing first services and he reckons cant be done and also said its not to big a deal not doing gearbox oil

I would beach the cat with the right tide. let the motor/s hang in their operating position (they should not hang lower than your mini keels).
You can get a small 1 liter oil pump from Supercheap or elsewhere and an iceacream container for the waste oil. one spanner for the two plugs and your in business. Something under the leg to catch the plugs if you drop one would be good too.
I understand that the dealer service is not compatible with the above but you could lift the o/b into the cockpit and have it done there or take it to the dealer.
These motors are easily serviced in-situ on a cat and it will save $$$$$$.

these engines are pretty heavy, for sailing I can lifted and tilted , for oil change needs lifted only.
my guess ,beach the boat , get ready , means I use big syringe or plastic tube taped to the top of the oil bottle.
now I changed the gear oil after a couple of years.

If replacing the oil from the dinghy or beach, it would pay to havea couple of spare fibre washers that go under the screws on hand, just in case one gets torn when you remove the screws.

I do the work on my motors in the water but then my nacelles pivot. I don't think you will get the motors vertical if you pivot into the dinghy but you don;t really need to for draining. You may for filling though.

Beaching is a really important facet of owning a cat. Choose a nice flattish beach out of the wind and waves preferably. A nice big flat oil tray underneath and plenty of rags and a plastic bag should be good for cleanup.

I like my Yammie 9.9, it is a good bit of kit. I would also consider buying an oil changer from Bunnings. It does well with a smaller tube taped on for the 9.9 motor sump oil. Much better than draining from the plug. Do this often and your motor will be nice after a decade - mine still is purring along.

www.bunnings.com.au/briggs-stratton-oil-removal-kit_p3370025

cheers

Phil