Thank you, Jane and Jim, for your advice.
Just to clarify, the warm then cold stratification regime is recommended
(apparently for all western lilies) by Alan Bradshaw of Alplains, who
sold me the seed:
Warm, moist stratification required for 3 to 4 months, followed by 2
to 3 months cold stratification during which germination takes place
while cold. Expose to warmth when germination is complete.
This does make some sense from what I understand of the climate in the
Siskiyous and of "delayed hypogeal" germination: the seed actually
germinates in response to late summer rains, when it forms not a leaf,
but a tiny bulb which remains dormant until snow melt, when the first
true leaf appears.
But as the Archibalds say:
Seed germination has been categorized in most books on lilies but
the lilies have not always read the books so do not trust such
information implicitly.
Max