Where are all the super-AGB stars?

I've been following with interest the progress that has been made at
understanding the fate of stars in a certain mass range, probably
within 7-10 Msun, which burn Carbon but never proceed farther. It is
apparent that at least some of these stars (except possibly at the
lowest metallicities, which no longer exist) will lose enough mass to
become an ONe WD (and many WDs in that mass range are known, all 1.1
Msun must be ONe, unless they later accreted additional mass), and at
high enough metallicities, essentially all.

There have been several supernovae attributed to such stars but no
unambiguous detections of the stars themselves. Their track before
carbon burning has no qualitative difference with those of lower or of
higher mass, so we must not look there. It is the later, mass-losing
phase that we must consider. Theoretically, these should be
distinguishable by their place in the HR diagram: they are redder than
any supergiant (the tip is ~M8, compared to the reddest SGs at M5),
but brighter than ordinary AGB stars - the tip of the AGB is ~30,000
Lsun or Mbol ~ -7, while a super AGB star can reach up to ~100,000
Lsun or Mbol ~ -8 (from memory, anyway - it's something like that).

However, it seems these thermally pulsing stars will be obscured by
dust. I'm not entirely sure why, given that lower-mass giants are not,
but that seems to be from what I've picked up. Is there any hope of
detection? One indirect method would be seeing a SN with a known
progenitor which was enshrouded in dust (which SGs are not), but that
very rarely happens.

Andrew Usher