1) The search function for the list appears to only work for the most recent few months.
2) I asked this question to the list in 1999 and it also came up in some other discussions about Gayatri mantras later on (2019 )
The replies I got onlist and offlist were:
a) As
for Whitney, I am not sure when such a shift in the understanding of
dhīmahi begins.
I have a suspicion that even in the old famous Gāyatrī:
bhargo devasya dhīmahi, dhiyo yo naḥ pracodayāt, the poet is
playing with the relationship between the verb dhīmahi and
the subsequent foot: dhiyo yo naḥ pracodayāt, and hence the
understanding of dhīmahi as
alternatively being derived from dhī may be quite old. I was
struck more by the almost sustained shift to the use of dative with
dhīmahi and
vidmahe, and have been thinking about what might have prompted this
shift, in spite of the fact that much of the later formulaic use is
in imitation of the old famous Gāyatrī mantra that uses the
accusative.
b) Hoffman
treats this particular form in his AufsAtze sur Indoiranistic, bd
2,
1976,
pp.483-485
c) As
for dhImahi. It is indeed optative, not
of dhI but of dhA, see again K.Hoffmann’s Aufsätze somewhere (via
index). Again both Witzel-Gotō and Jamison-Brereton recent
translations have it right: “we wish to place” (no ‘meditation'
here, that is a later interpretation).
d) dhīmahi was
an aorist optative (or, less probable, injunctive) in the RV: 'may we
obtain / make our own'. On the one hand, these forms become rarer and
rarer already in Vedic texts, but may still have been understood by
some (as we still understand forms such as 'maketh', but do not use
them). On the other hand, dhīmahi cannot
actually be a present form: it would have to be dhīmahe. As of now I
do not know when and how the reinterpretation occurred first; the
texts I've surveyed so far are generally silent on the
individual words of the mantra. However, the parallelism with vidmahe
in the modified Gāyatrīs may point to a more "cognitive"
interpretation.
e) I've
just checked Werba's Verba Indoarica for dhImahi (under
dhA) page 298 and he has this as an aorist optative.