Reduction or put jelly directly on the bird?
I think your first attempt could go either way, dictated primarily by your own personal taste.
Myself, I don't care much for overly sugary meats, which are popular in many American restaurants, so I would probably opt to thin out the jelly with stock, water, vinegar, citrus juice, etc. The latter two provide some acidity that would offset the sweetness of the jelly.
However, if you love sugary meats, then go for it, brush on the jelly at full strength, "Damn the torpedos, full speed ahead!"
It's really your call based on what you envision the final dish will be.
There's no way in hell I'd brush on full strength lemon jelly, but for certain my taste buds often don't match with what's popular with the typical American consumer.