Should the p5 community arrogate to itself the job of spec'ing a p6 site architecture?

If you write a Perl 6 site architecture specification, you're by definition part of the Perl 6 community, at least a bit. Especially if you actually start to implement it.

And even if you don't, there's no harm done. Either somebody picks it up and tries to implement it, or not. By writing a spec you don't force the Perl 6 community to do anything at all.