OK, here's an overview of the problem:

http://www.pbase.com/robertbarr/image/107748863

This is the result of two separate leaks over time. It's a finished
basement bedroom, and was a pretty low priority, so it languished a bit.

Home was built around 1940, usually top-quality construction throughout
- with exceptions.

Here's the problem:

http://www.pbase.com/robertbarr/image/107748871

The brass tube heading upward is from the tub overflow. There's a Tee
in there, with a pipe heading back to pick up the tub drain. Note that
the framing lumber was relieved to make room for the plumbing.

As I noted, there was another leak which caused a lot of the white
discoloration. That leak was repaired a long time ago. This current
problem is the one I'm addressing, and it obviously begins at that Tee,
where the pipe heads downward toward the trap.

Here's a much better view of the bottom of that Tee: (kinda large file)

http://www.pbase.com/robertbarr/image/107748935

Unless there's something really wrong with my eyesight, there's no
compression fitting on the lower outlet of that Tee! There's obviously
one on the upper joint (where there was plenty of room), but none on the
lower. I'm guessing that the builder got tired of 'relieving' the
framing member, and there just wasn't room for the fitting. (Also no
way to get a wrench in there to tighten the fitting). Really
substandard work. Not to speak ill of the certainly-dead-by-now, but --
What a moron.

I'm looking for a solution here. My primary plan is to remove the whole
trap, use a sawzall at the lower right of the first photo, install a new
fitting (PVC probably) into the brass Tee, create enough room to tighten
the fitting this time, and somehow join the other end to the cut drain pipe.

The pipe heading into the Tee measures 1.5" O.D. The O.D. of the drain
pipe where I plan to cut is 1.75". If I use PVC, how the heck am I
going to make the junction?

I saw a halfway similar segment on Ask This Old House, and Richard used
sort of a bellows type of flexible coupler that used hose clamps. How
would that work with two different sizes of pipe?

(Yeah, also planning to reinforce that framing member, even though it's
held up fine for 68 years; it's also holding up the tub!)

Ideas welcome. Thanks.