I am thinking on my upcoming YYZ - BKK trip to do AONE3 and go west on AA, then CX to HKG and BKK then go home on QA or BA from BKK to LHR and then to YYZ. To try out a few carriers F class and so forth and frankly conencting in London might include a day there or two since I haven't been.

When booking flights are "A" seats generally available or is it akin to award seat travel with its limited "C" and "R" and "X' seats? I'd hate to want to try changing the route (or even trying to set the itinerary at the outset) which I understand is permitted after you start and then find it is impossible to find seats.

The YYZ-BKK route on CX in F (assuming they have F when I go which they don't) costs more than double AONE3 so obviously AONE3 is a good deal if it can get me there and back when I want to go.

There's no easy answer. Some airlines/routes offer no or very limited A availability (e.g. some hot AA domestic routes) but on BA and CX I've generally had little trouble. Not "none", though. And of course it depends on how far in advance you're asking. If it's for tomorrow, it's the luck of the draw. They're treated as revenue seats, though, seemingly way ahead of awards. Look at your favorite booking engine for the airlines/routes you're interested in to get an idea.

A inventory is reasonably available (much more than award inventory) but some routes sell out in full-F on some days of the week/month. BA is doing a "buy J, fly F one way" promotion which is really filling up the F cabin in advance this year. It is definitely harder to get A seats than a year ago, but you'll have no trouble on the routes you mention with a tiny bit of flexibility.

Good point -- but remember that you can ask for a seat to be released for you, and the AA RTW desk is remarkably successful in getting seats released on some of these routes. The problem with A on AA is that A is also used for upgrade inventory which distorts availability on the hot routes (but AA does distinguish between revenue A and upgrade A, somehow).