There is always the possibility of getting the 'ideal' values wrong, eh? 
So, now the offsets are linear, I'll calculate a compensating equation for it. Sorry for the bother!
But this approach seems to work and may be interesting for others to use also, i.e. marrying a inexpensive MUX like the 4067D with a nice ADC to get a lot of channels. Add three precision resistors and all sorts of things can get compensated for, allowing you to use higher-tolerance resistors / components elsewhere.