Douglas Krupka refers me to the following:
Although in discussion of the imperative clause type it is routinely denied that it could ever feature a past tense, imperatives in the past tense do exist. Specifically, past imperatives can be found in (Northern) Dutch and Frisian, many speakers of which can produce and understand sentences like (1) and (2).
(1) At liever eens wat minder! (Dutch)
ate rather once somewhat less
The English equivalent seems odd to me, but you would think it is hard to translate into a language which does not have imperatives in the past tense. Best to put the English out of your mind and focus on the:
At liever eens wat minder!
You can do a Google search on the concept here.