Just out of curiosity I rang two lighting consultants for technical
info. They both said the same thing - yes, you can use a dimmer
on a halogen bulb.

Normally the bulbs run so hot so that tungsten is re-deposited on
the filament. What happens when you dim them is that the re-
depositing cycle is broken, BUT the life of the filament can be
greatly extended because of the lower average voltage through it.
So although less tungsten goes back on the filament there's a
corresponding lack of need for it to do so because of the lower
filament temperature.

The colour will quickly go from bright white to yellow as you dim,
as an ordinary filament bulb would do with a dimmer. "Soft start"
dimmers are recommended, the type depending on how the
bulb voltage is derived, ie a wound or "electronic" transformer.
I assume "soft start" implies zero-switching.

When winding up from dim to full power, there's a delay to full
brightness until the quartz heats up and the tungsten left deposited
on it can escape back into the bulb atmosphere.

So there you have  it.....

Jinx

PS I'm not a fan of dim yellow lighting and wonder if a fine metal
screen could be put in front of the bulb. That would give a grey
light rather than yellow. Neither lighting people had heard of that
being done. Fine venetian blind ?