I've talked to a service writer about this issue. It's a dilemma for them. If you bring in your own parts, say a timing chain, and it fails two months later, who is going to eat the labor for another replacement? The place you bought the timing chain from might warranty the part, but that won't include the labor that the dealer does. If you could prove the failure was caused by improper installation by the dealer, well yeah, you might have a case, but it would be a tough one to prove.
So looking it from the perspective of the dealer, they run the likelihood of pissing off a customer doing this sort of work, so probably the safest thing to do is to avoid it whenever possible. Or at the very least, for dealers willing to accommodate this sort of request, to make sure that the customer understands the ground rules up front.
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