Hi Jason,
I've attempted a similar task with limited success. In my experience the annuals - wheat, buckwheat, peas - just couldn't compete with the already established sod. The white dutch clover, being a perennial pasture plant, will probably do well if the other grasses are kept shortish until it is established (clovers are great chicken feed too!). Careful with Alfalfa - it might do all right in the growing season but is really intolerant of waterlogging in the winter, and I assume in Langley you're on floodplain.
If I were you, I would attempt some sort of shallow tilling. If your soil is sandy - as the gravelly subsoil indicates - you may not need to deep till at all. Annuals are opportunistic species that require a disturbance to become established - however, without killing the sod it'll eventually displace the annuals.
If you're not keen to till, look at other perennial species that might establish in a pasture. White clover is a good bet. There are perennial phacelias, and I'm sure plenty of other flowering perennials. Look around and see what 'weeds' are growing in local farmer's pastures - they might be appreciated by your chickens and add diversity to your pasture. Also, consider the grass types you've got. I can't say from experience, but I suspect softer grasses like orchard grass would be more palatable to chickens than fescue or reed canary.
Best of luck, and keep us updated! I'll be seeding some chicken forage this fall and would be interested to see what works for you.
