Pimperial said:
Despite having ridden mountain bikes for most of my young life and nowadays tackling the roads pretty frequently, I've never been able to ride with no hands. I always feel very unstable, possibly because I'm 6'5" and 15 stone perched on a saddle as high as the seatpost will allow.
Is this top-heaviness a factor or do I just need to try harder to keep balanced with no hands?
Top heaviness has absolutely nothing to do with it.
I'm 6'2" and used to be thirteen stone, and have never yet found a bike I couldn't ride no hands. I can take my jacket off, roll it up, and stick it in my pocket while riding. It's also quite handy to be able to stretch while riding.
Obviously the more directionally stable a bike is the easier it is to ride no hands (e.g. touring bikes); the more twitchy the steering, the harder (race bikes). But every ridable bike becomes more more stable as it gets faster, so all that really means is that the minimum speed at which it is comfortable to ride no hands gets higher.
As to safety, don't cross white lines no hands, and be wary of any other imperfection of the road surface which could twist the front wheel, and you'll be fine. Yes, it isn't a very safe thing to do and I wouldn't do it in traffic, but it isn't dangerous either. I think practising improves your balance on the bike generally.