a database dump of the PM database (with the passwords and other sensitive information nuked, of course). That way i could start to design the PG schema and write a proof-of-concept importer for the user accounts and posts.

Well, how about let's instead start with PM's MySQL schema, and understand how the tables all work and tie together. There's lots of stuff there which would not need to be duplicated in the new system.

Getting "feature complete" on par with existing PerlMonks with all the bells&whistles and finetuning might take quite a bit of work (polls, XP, APIs, etc).

Exactly. That is stuff we can add once the base system is up and running.