Pardon if I haven't lurked enough to find this answered elsewhere (only went through one page of results...).

Recently bought World of Goo as part of the Humble Indie Bundle, but alas, I can be a whore for little badges that say I did things, as such with Steam.  I've downloaded the demo for that platform, and was wondering if I could copy the pers2.dat file (or append whichever equivalent for Steam), and if so, how.

[aslo, frist prost!]

I think you have to by directly form Steam if you want to enable steam achievements. Also, you have to have steam account.
But it might work, pers2.dat file is located in the same place for demo and full game... I think.

At what point is pers2.dat made?  I entered a name on the demo, to see if the file cropped up in the places I was looking, but nothing.

pers2.dat is made when you enter your name in profile slot and hit enter. Did you look at the right place? I didn't remmber where Steam keeps pres2.dat file, try '/Program files/Steam/World of Goo' 
pers2.data file from the Indie Bundle pack game should be in 'Documents and Settings\YOURNAME\Local Settings\Application Data\2DBoy\WorldOfGoo'

Today I visited Steam.com and I'm not sure your plan will work. Achievements are enabled when you buy the game from steam, not just download demo