I don't see why a right navigation is considered unaccessible.
The document structure is the same for both left and right navigation.
(in my case anyway)
Textbrowsers won't notice any differences, nor should any other type
of browser?
I think the concern is that it may be problematic for people with certain
mental disabilities or speech/language problems, simply due to its being in
an "unexpected" place. For example, people with certain autistic-spectrum
disorders may find it quite stressful to have their usual routine broken
(and, let's face it, even users without disabilities are likely to
experience momentary stress when they see something that should look
familiar arranged in an unfamiliar way. It's something that needs to be
adjusted to and while most of us can make that adjustment pretty quickly,
it can be rather effortful for some people).
Or someone with a physical disability that makes mouse/keyboard use
difficult (but not impossible) might be running some sort of utility that
allows them to "jump" the cursor to particular areas of the window. In
that case, they'll probably have a "mouse macro" to put the cursor at the
left edge and a little bit down from the top, since that's where the
majority of navigation menus live and "homing" the cursor there reduces the
amount of mouse motion needed to navigate. Putting the menu on the right
means they have to manually scoot the cursor over to the right edge and
define another "macro" that's just for this particular site's navigation.
Accessibility isn't an all-or-nothing proposition. In both cases, the
combination of the user's disability and the handicap [1] created by the
non-standard layout doesn't make the page completely unusable, but it makes
it substantially less usable (more mental or physical effort required).
[1] Terminology:
Impairment: "something wrong with" a user's brain or body.
Disability: finding a task difficult or impossible as a result of an
impairment.
Handicap: a feature of the user's environment that makes an impairment more
disabling.
Example: Color-blindness is an impairment. It normally creates little if
any disability. But if a page presents all its links with poorly-
contrasting foreground and background colors, it presents a handicap to the
color-blind user. In this case, a handicap that creates a disability out
of thin air.