  (Abridged) Among the various issues that remain open in the field of
accretion onto black hole X-ray binaries (BHBs) is the way the gas accretes at
very low Eddington ratios, in the so-called quiescent regime. While there is
general agreement that the X-rays are produced by a population of high-energy
electrons near to the BH, the controversy comes about in modeling the
contribution from inflowing vs. outflowing particles, and their relative energy
budget. Recent Spitzer observations of three quiescent BHBs have shown evidence
for excess emission with respect to the tail of the companion star between 8-24
micron. We suggest that synchrotron emission from a partially self-absorbed
outflow might be responsible for the observed mid-IR excess, in place of, or in
addition to, thermal emission from circumbinary material. If so, then the jet
synchrotron luminosity exceeds the measured 2-10 keV luminosity by a factor of
a few in these systems. In turn, the mechanical power stored in the jet exceeds
the bolometric X-ray luminosity at least by 4 orders of magnitude. We then
compile the broadband spectral energy distribution (SED) of A0620-00, the
lowest Eddington-ratio stellar mass BH with a known radio counterpart, by means
of simultaneous radio, optical and X-ray observations, and the archival Spitzer
data. We are able to fit the SED of A0620-00 with a `maximally jet-dominated'
model in which the radio through the soft X-rays are dominated by synchrotron
emission, while the hard X-rays are dominated by inverse Compton at the jet
base. The fitted parameters land in a range of values that is reminiscent of
the Galactic Center super-massive BH Sgr A*. Most notably, the inferred ratio
of the jet acceleration rate to local cooling rates is two orders of magnitude
weaker with respect to higher luminosity, hard state sources.
