It's a far from satisfactory charger anyway, because it mostly charges above 0.1C with no termination, relying on user intervention to terminate the charge in accordance with the time periods stated in the user manual, based on cell type and nominal capacity - and of course that assumes fully discharged cells, with no consideration being given to the possibility that they might still have a partial charge, so over-charging is always going to be a high possibility for one reason or another.
If the charge circuit is a true constant current generator then in the event that a single cell was inserted with a shorting link to complete the circuit, the cell would charge just fine, at the design current, but the other 1.5V or so would then have to be dropped across the series resistive element, be it a constant current IC, or a voltage regulator or transistor configured as a constant current generator - and that extra voltage might take the series device above its power dissipation limit. The series element might be just a resistor, combined with a suitably high DC supply voltage to approximate a constant current but the same thing applies - the extra voltage dropped across the series resistor might take it above its power rating.
All things considered, it hardly seems worth the effort because sooner or later it's going to damage your cells even if it's used as intended, or if used to charge single cells then the charger itself might end up being damaged.
If you intend using it for single cell charging anyway then I reckon the safest option would be a dummy cell containing a series resistor calculated to drop 1.5V at the designed charging current, and with an appropriate power rating.
