Analysis of RTs when speed-accuracy trade-off leading to ceiling accuracy has occurred I know that it is not desirable for behavioural data to show a (within-subjects) speed-accuracy trade-off, as it can lead to e.g. the wrong interpretation of a single subject's decreased RTs as meaning improved performance, when in fact if one looks at error rates, one can see the increased speed came at the cost of increased error rates.

However, what I don't understand is why, assuming that subjects are instructed to respond slowly enough so that their error rates approach zero (so not the usual speeded responses paradigm), why would it not be correct to then analyse the group RTs, which will probably show wide between-subjects variations and can thus offer meaningful interpretations? This way, it would not matter that all subjects will have traded speed for accuracy, as it is precisely the amount of "slowing" that becomes the dependent variable.

Hope the question makes sense, thanks!
You may be able to reduce errors by telling participants to react slowly. However, by adding this instruction, you may also dilute the response time differences that your are trying to find (whether between or within subjects). If you are lucky, you are just adding random noise, however it is more likely that you will add confounds or tap into different mental processes (occuring at a later time). 

Another option to tackle this problem is to hold the speed more or less constant, by giving participants a very narrow time window to respond (which can also be tailored to their speed). As a result, they will produce more errors, which now become the dependent variable (see Draine &amp; Greenwald, 1998, who have introduced this technique).

References

Draine, S. C., &amp; Greenwald, A. G. (1998). Replicable unconscious semantic priming. Journal of Experimental Psychology. General, 127, 286–303.