Zag - I agree that asking for seiyo wasabi or fake wasabi would likely
come off as cheeky or even rude. I don't, however, think it an
affront to ask for the real deal, but have observed some curious
reactions or embarrsment amongst sushi chefs at establishments that
either don't know or acknowledge the difference or whose owners decide
that they don't want to support such. This scenario is not exclusive
to wasabi by the way... I've experienced frequent substitution of
masago for tobiko and even had one chef argue they were the
same...then there is "kani" which is almost always "faux".

JS - I believe the polite way would be to quietly ask if real wasabi
or hon wasabi is available. You may still elicit a raised eyebrow,
but that may be because they are surprised (but not offended) that you
know the difference. While still not available at most restaurants,
even some of the best, the word is spreading, largely due to consumer
awareness and more frequent requests. Real wasabi is also
increasingly showing up on more menus, typically with an upcharge of
$3 to $5 per serving, however, quite a few chefs still keep it under
wraps in the fridge for select presentations, to delights special
customers or to provide for those that request it. If you don't ask,
you might never know...on the other hand if you do, you might be
pleasantly surprised.

Clearly costs are a driving force in these scenarios, but IMHO
consumers should not have to be sommeliers or gourmands to justify
their pursuit of authenticity.