I think when it comes to flying aircraft home empty I assume you are referring to the required two local nights when diverted on long range, I think this agreement should be looked at and it will be because cabin crew just want to get home too !!
Our problem with working one / two down from base is, if we agreed to do this it would happen all the time as for some reason BA never manage to get cabin crew crewing levels right and end up being short of crew !!
As for working crew down from overseas we've always been able to do this and down to minimum crew IE. 12 on the 747 and 10 on the 777 and 6 on the 767. so unless there is an issue with getting two local nights we do not have flights operating with zones closed or empty back to base. I and many of my fellow cabin crew do this on a fairly regular basis due to sickness etc, I'm in HKG and will be working one down back to LHR, in Jan I operated SIN-SYD-SIN one down due to a shortage of locally based crew in SIN.
We did go through a period of "ferry" flights to destinations with pilots only but that was nothing to do with the cabin crew agreements it was because BA manpower got it wrong with the number of cabin crew they needed on worldwide, thankfully they have it right at the moment but are predicted to be short again in the summer !!
Quite often BA elects to "ferry" without seeing if the cabin crew can have an alleviation to work one down from the unions
Sadly there have in the past been cases where the cabin crew have been flexible and been given alleviation to work outside agreements with the promise of some sort of payment only to find our management have done a u turn and decided not to pay, it is any wonder cabin crew are very reluctant to be flexible with this type of history from our managers ?
As for flight crew leaving one down I must admit I've never seen this, although you do have a forced draft system which I guess helps, and I'm pretty sure your management don't do a u turn when it comes to the compensation for working one down or forced draft ?
The reason we have such ridged agreements and they are collective so the BA has agreed to them as well,this is because we have a max of 16 individuals have personalities from being analytical to the hysterical and it would be impossible to get everyone singing from the same song sheet without the agreements in place.
When it comes to the flight crew there's only 2,3 or 4 who generally are fairly analytical and singing from the same song sheet !!
One of the things BA want is to flex not only our trips but our days off too and unlike forced draft there will be no compensation for having to work on a planned day off !!
This means if you check in for a HKG and they are short on the BOM they move you onto the BOM to prevent a stand by delaying the BOM. This will not only flex the trip but the days off as well and subsequent trips will be flexed indefinitely so you can never plan days in the UK !!