How can I construct a jig for drilling and placement of legs on chairs that allows you to replicate both backward and sideway splay of the legs? Or maybe even point me in the right direction if there's an existing one that's commercially available? I do it by hand now and it is a bear to duplicate without a jig. I need a method, jig or at least some advice.
Are you talking about something like this?
If you have access to a miter saw or table saw, it should be fairly easy to make the compound cuts for the guide block. Then keep the shaft of the bit against the corner while you're drilling. You could even use this rudimentary jig to create another hardwood jig with an actual hole in it that can be clamped to the work piece. That way you wouldn't need to worry about the bit wandering out of the corner in the guide blocks.
The author of the blog this is focused on hand woodworking, but it could easily be adapted for a powered drill setup as well.
Thomas Mosers's Windsor Chairmaking (Blog author lists this as original source)