Not quite direct, but I've known two people who kept regals in a mixed reef, but neither with clams. One was a special case I won't go in to, the other had a very large tank (500 gallons?) and fed heavily a couple times a day. Neither had nipping issues with their coral.
That said, there are three ways that I've seen fish develop a taste for something:
1. You get lucky, and they never do.
2. They get curious and nip a bit here and there. Damage occurs over a long period of time and may be acceptable or at least give you time to remove the predator or prey.
3. They act like a kid who wakes up in a candy store. One of the start up uglies my 600 reef had was green ulva, which was roughly 1" thick through the entire display. One evening my 6" desjardini decided it tasted good, having never previously touched it. When I got home from work around 7 PM the next day the tang had the fullest stomach I've ever seen and the entire tank was shiny and white. Not a missed spot in the corner, not some hint of it down in a groove, barren. It was still growing in the sump, and a bit in the overflow, but the entire display had been scrubbed clean. I wouldn't believe a fish that size could eat so much if I hadn't seen it myself. I knew a person back in DFW who watched an angel eat a basket ball sized colony of zoas in 2-3 days after not touching it for at least a year.
My point being, if one day your angel does decide it likes clam, there may be zero warning.
