32dev.sh with sbopkg...
 

Okay I'm mainly asking beforehand so I won't have to shoot myself in the head twenty minutes from now.

I suppose I'll find/figure out one way or the other anyway ;)

Can I just source the 32dev.sh and then run sbopkg -i if the source for the package I want to create isn't 64bit capable?

Oh wait. I might need to change the $ARCH variable to x86_64 too, right?

The SlackBuilds need to be modified

See:
http://alien.slackbook.org/dokuwiki/...2-bit_programs

Also sometimes needed:
http://slackbuilds.org/faq/#multilib

sbopkg allows you to customize SlackBuilds if you run it interactively.