Looking for ideas to fix this. Would like to avoid a lot of work under the slab if possible. Could I seal it up somehow and put a clean out in the ABS above?
You're not likely to get anything to "stick" to that cast.. and if that portion looks as bad as it does, then likely the rest of the fitting is in as bad shape. Hard to say, but you might be better biting the bullet and getting this repaired on your schedule rather than an emergency when it collapses.