  We performed optical spectroscopic monitoring of the BL Lac object AO
0235+164 (z=0.94) with the VLT and TNG telescopes from Aug. 2003 to Dec. 2004,
during an extended WEBT campaign. The flux of this source is both contaminated
and absorbed by a foreground galactic system at z=0.524, the stars of which can
act as gravitational micro-lenses. In this period the object was in an
optically faint, though variable state, and a broad Mg II emission line was
visible at all epochs. The spectroscopic analysis reveals an overall variation
in the Mg II line flux of a factor 1.9, while the corresponding continuum flux
density changed by a factor 4.3. Most likely, the photoionising radiation can
be identified with the emission component that was earlier recognised to be
present as a UV-soft-X-ray bump in the source SED and that is visible in the
optical domain only in very faint optical states. We estimate an upper limit to
the BLR size of a few light months from the historical minimum brightness
level; from this we infer the maximum amplification of the Mg II line predicted
by the microlensing scenario. Unless we have strongly overestimated the size of
the BLR, only very massive stars could significantly magnify the broad Mg II
emission line, but the time scale of variations due to these (rare) events
would be of several years. In contrast, the continuum flux, coming from much
smaller emission regions in the jet, could be affected by microlensing from the
more plausible MACHO deflectors, with variability time scales of some months.
