You can...the rule is meant to be that you can't MFU a ticket which wasn't paid for/issued in your home country (i.e. I can upgrade AMS-LHR-NYC provided I buy it through BA UK as I'm a UK member). Some signs of various exceptions here (yyzlhr of late IIRC).

I think this is a good question. I've always found availability to NYC good and that's just long enough to make it worthwhile IMHO (plus the fact you get off first for immigration). DFW in particular seems to have wide open availability (but the base T fares are nothing too special these days sadly). YVR (off season, e.g November for me) was wide open and a good use of miles IMHO as it's ~9hrs each way. I'd probably not bother with BOS as that's rather short (unless the T class was say <£500 for some reason and I was going anyway). Never done one to the Far East, but agree HKG would be a great use given its length/timings.

Phil