Chances are that the old and well-used 2700s have developed high internal resistance, and either the charger is testing for that and indicates 'full' as an alternative to showing an error, or the high resistance is fooling it into premature termination.
Going high resistance is pretty much a one-way process, even if they may still be quite usable for some purposes. There's a good chance their self-discharge rate is fairly high, but that's not a great issue if they're going to charged and used immediately, especially in things like well-used radios where they're not really needed to hold charge for months.
Pending some serious improvement in Li-ion capacity, I wouldn't hold my breath re: 1.5V Li-ion AAs. Li-ion cells have a naturally much higher voltage and to give 1.5V in an AA package, they need voltage conversion circuitry built in, which means the actual cell would be somewhat below AA size, with correspondingly reduced capacity.
There were cells like that around a few years ago, but they were expensive and needed a special charger. As they had lower overall capacity, they didn't give any obvious advantages over decent (and much cheaper) NiMH AA cells apart from novelty value, except possibly for the odd piece of badly-designed equipment which only worked if cells were close to 1.5V, and so didn't work properly on NiMH *and* wasted much of the capacity when used with alkalines.
Also, because they gave a regulated output while operating but shut off when the internal cell hit its minimum voltage level, they wouldn't give the relatively graceful end-of-discharge behaviour that NiMH cells tend to have things like lights, but essentially just turn off.
Even if they cost the same as NiMH cells, were as robust, and had somewhat more capacity, down-converted Li-Ions giving 1.5V still wouldn't be my choice to power a caving light if they shut off without warning.
