Sounds feasible. Apart from one question - how does it make any difference if the extra weight is carried in the hold or in the cabin?
If it's just a matter of mass then every passenger should be given an allowance of 30kg which can be split between hold and cabin baggage, with the restriction that cabin baggage must be one or two bags small enough to easily fit into the overhead lockers.
It makes alot of difference. I've been on flights where we hit extreme turbulence and the overhead compartments opened up and things started to fall out. Do you really want a 30 lb bag falling towards your head. The occurrences may be rare but once is too many times. Have you listened to the announcements prior when boarding, the overhead compartments are for lightweight articles only. Fuel is expensive but the cost that airlines pay in insurance is atrocious and if someone even put an overweight bag in the overhead they are not the one that will be sued if it falls on someone's head. No one likes to put much under the seat in front of them and I just shake my head with the things that people try and stuff in the overhead. One thing that slow down an arrival more than anything is the time that it takes many people to retrieve their bags from the overheads.
I used to design flight schedules for airlines in Canada. If we could shave 5 minutes each turn an aircraft did then maybe we could fit in another whole flight by the end of the day. It is a complicated process with the basics is that you can't make money with them sitting on the ground. We used to alot 35 minutes for a narrow body (737, 319, 329 types) from the time an aircraft arrived at the gate until they pushed back. Widebodies doing international flights will get up to about an hour 15 depending on the route and airport in question. In that time we had to offload everything, clean up the mess left behind and load everything for the next go round. If we could speed things up by limiting the amount of things people carry on then that is what would be done.
Normally the reduced carry on weights such as 7.5 kg is done on international long haul flights. Domestic operations in north America have much higher allowances and agents tend to look the other way alot. Trying to keep the weights down are important when you take a rather large aluminum cylinder and keep it aloft at 35,000 feet for up to 16 hours.
The rules are there for the safety of both the public and crew and aren't instituted willy nilly. I see it as somewhat of a hardship now that I am traveling with alot of gear but I make my plans based on the rules and if that means that I ship some of my gear ahead of me then that is what I do. The situation posed by the OP here hardly constitutes that sort of extreme measure and he should be able to get that to within half a kilo of the limitations. There are always those that post that they have never been challenged at the gate and to those I say you have been lucky. I have been at gates when people had to surrender some of their cabin baggage to go in the cargo hold. It brings some people to tears, which doesn't work on most gate agents. Everyone has a story why they should be exempt and everyone has expensive items in their carry on. You can try and work your way around the rules but don't cry foul if you get caught.

google "british airways profit margin"...
