  Luminous blue variables (LBVs) are expected to play an important role in
massive stellar evolution as well as being the progenitors of some of the most
luminous supernovae known. In this paper we provide a multiwavelength study of
the population of (candidate) LBVs identified within M33. New spectra provide
an observational baseline of >4yr with respect to published data, which is well
suited to identifying LBV outbursts. Multi-epoch optical and mid-IR surveys of
M33 further constrain the variability of the sample and permit a search for
dusty circumstellar ejecta. Spectroscopic and photometric variability appears
common amongst the sample, although in many cases further observations will be
required to determine its physical origin. Nevertheless, we report a new
outburst of M33 Var C, while the transition of the WNLh star B517 to a cooler B
supergiant phase between 1993-2010 confirms an LBV classification.
Proof-of-concept quantitative analysis is provided for Romano's star; the
results being consistent with the finding that its bolometric luminosity varies
during its LBV excursions. The combination of the temperature and luminosity of
two stars, the B hypergiant [HS80] 110A and the cool hypergiant B324, appears
to be in violation of the empirical Humphreys-Davidson limit. Mid-IR
observations demonstrate that a number of candidates appear associated with hot
circumstellar dust, although no objects as extreme as Eta Carinae are
identified. The combined multiwavelength dataset suggests that the population
of LBVs studied is contaminated by stars demonstrating the B[e] phenomenon. Of
these, a subset of optically faint, low luminosity stars associated with hot
dust are of particular interest since they appear similar to the likely
progenitors of SN 2008S and the 2008 NGC300 transient, albeit suffering less
intrinsic extinction. [ABRIDGED]
