As a newby to ipv6, i've just discovered that its not reliable to only
set a subdomain to a AAAA entry : because most of ISP will not be able
to reach it
This is likely true, but it's not a DNS issue. The problem is that most people don't have routeable or brokered v6 capability yet, so they can't talk natively to ipv6 hosts. No amount of advertising weirdness in your DNS will give hosts that have globally-routeable v6 addresses, but only privately-routeable RFC1918 v4 addresses, automatic global accessibility from v4-only clients. Your only choice for making your hosts accessible to v4-only clients is to enable good old NAT punchthrough on your firewall, and use port numbers to differentiate your hosts.
Don't worry; IPv6 is coming. My chats with large UK ISPs a year back suggested that they intended to start rolling it out around 2014, and some of the smaller, cleverer ones have it now. I myself now have a routed /56 to my house (I haven't quite worked out what to do with 4.7x10^21 addresses yet; I'm currently using four of them), although it meant changing the way I was connected. What you're doing will help, in a very small but nevertheless important way, to add to the global pressure for v6 to the house; once that comes, your public-v6-but-private-v4 hosts will be globally accessible.