So it looks that it just works. GCC provided by Sun which starting from b74 is included with Solaris just compiles both 32bit and 64bit code. MySQL compiled as 64bit binary and passed run-all-tests. If I will have enough free time I will try to compile MySQL with Sun compilers and compare run-all-tests with GCC - would be interesting.
I installed Perl and some modules from Blastwave on b74 64bit in a local zone - and it just works, I mean it installs without any problems and without any difference comparing to system without zones, then you just run and that's all! IIRC These packages are compiled for Solaris 8 and still they just work on Solaris 10 - I've just love Solaris backward compatibility.
If you have any suggestions what to test on the v20z please let me know - if there will be enough time I will try at least. But I'll have to give back the v20z in a couple of days so please hurry.