I'd prefer to hold Mason out of the deal, but I definitely wouldn't let him be a deal breaker.
Get them thinking? Unless they're dead set against trading Iguodala, I'm not sure they could do better. Mason, more than an expiring contract, would be a useful piece for the notoriously poor outside shooting 76ers. They have one knock down shooter on the roster, Kapono and he's now fallen out of the rotation for the time being. Even Bonner they could potentially use for this same reason. So two guys that they could actually use, a huge expiring contract (Jefferson) that will be a useful asset next season, plus Splitter and a 1st, which are both quality assets. That's a ton of value, just to move an overpaid non-franchise player (Iguodala) AND to get to dump a bad contract (Dalembert) in the process? I don't know if the 76ers could turn this down.
Spurs roster (with this hypothetical trade)
Starters: PF- Duncan, SF- Iguodala, C- Dalembert, SG- Finley, PG- Parker
Bench: SG- Ginobili, PF- McDyess, PG/SG- Hill, PF- Blair, SG- Bogans, C- Ratliff, SG/SF- Hairston
Inactive: C/PF- Mahinmi
Obviously, they'd need to add depth in the form of another SF-sized swing man who could knock down the three at a good clip and a run of the mill third PG, but as a base that's a strong, clearly defined top eight as a playoff rotation. I'm not big on Finley starting, but with with that starting lineup I'd want a 40% 3 point shooter to stretch the floor.